Adding A Second Router Doesn't Work Well

[Michael]

Both my modem (Alcatel Lucent i240G-T) and router (D-Link DIR-850L) are located at downstairs. In recent weeks, the signal has been weak when I'm trying to accessing the wifi from upstairs. I've tried to reset the router but it doesn't help. The signal used to be strong. Not sure what was the issue.

So I tried to use my old modem (ASUS DSL-N12E) as a second router upstairs. Now when I'm using the wifi upstairs, despite both network can be detected, the device (phone/laptop) will still "prefer" to connect to the D-Link network despite the ASUS router is located much nearer. Therefore, adding the ASUS as a second router basically serves no purpose as it doesn't work as it should.

Attached below is how I connect the three devices.

[Geek-9pm]

Here is a quick response.
I use double rooters in my home network.
In general, it not a recommended thing to do. But here are some reasons.
1. To extend the range of your network. Normal Ethernet cable go up to about 300 feet or about 100 meters.  A inexpensive router can be used as a 'boaster' to help drive another long cable.
2. You wireless coverage does not reach the cabin in the backyard. The same router in example above  could also extend the wireless range on a separate channel.
3. With two routers,you can have more  control over how guests use your wireless network.

To make two routers work together, you need a tutorial on how to set IP for each router and how to manage DHCP.
Have you already read this?
https://www.wikihow.com/Connect-Two-Routers

How to Connect Two Routers
Two Methods:Using Ethernet Using Wireless

This wikiHow teaches you how to connect two routers together. By connecting your routers, you can extend both the range and the maximum number of connections that your Internet can handle. The easiest way to connect two routers is by using Ethernet, though you may be able to use a wireless router to connect to the primary router

Some routers can be a pin. 

Now about your specific issue .Do all your devices have dual band options?
You need to  consider if that might be the problem.

In my case,I only use the lower band and I set both routers to use the same protocol, station ID and password.  Does that not work for you?

[Michael]

The secondary router (ASUS) is configured correctly, I can connect to it and the network is working fine, except I have to be very near to the secondary router (within a few meters). Otherwise, it will switch back to the primary network (D-Link) after a while.

Is there anyway I can check the signal strength from both the routers at one particular location?

[Geek-9pm]

When both routers have the same ID, your laptop must use the stronger signal. This is true of any other portable device. They automatically use the stronger if both are on the same channel # and have same protocol ID and password. The routers will tine-share the same channel.

As a test, try this:
Change  to names of both routers to be different. Do not use the old name again. This is a quick check to see if the device is weak. 

Abort signal strength.
Yes, some routers can show you the level of other devices in the neighborhood. D tails vary a lot. But here are some free things to try:
https://www.networkworld.com/article/2925081/wi-fi/8-free-wi-fi-stumbling-and-surveying-tools.html
Also:
https://www.raymond.cc/blog/homedale-wlan-monitor-watch-log-wifi-signal/

Watching how your WiFi signal behaves over several minutes or even hours could help identify if the current location for it is ideal or causing problems. Here we list 5 free tools that show a graph for your wireless signal so you can watch it over a period of time to see how it behaves.
1. NetSpot

NetSpot is a free wireless network signal analysis and troubleshooting tool available for both Mac and Windows computers. In addition to a standard WiFi discovery and monitoring section it also has a site survey feature that allows for the relative network signal strengths to be plotted onto a map of your building or local area.
Read More: https://www.raymond.cc/blog/homedale-wlan-monitor-watch-log-wifi-signal/

Try it. You might have a router with a damaged antenna.

[DaveLembke]

Additionally you can go into wireless settings and tell devices to forget the connection to the other weaker network connection so that it will only pick up on the stronger nearer connection.

I have multiple wireless networks in my home and tell the devices to forget the other connection and this way they only associate with the wireless that they have been set to associate with. If you keep multiple wireless networks in the devices list of authenticatable networks it will be nothing but problems!!!

If you decide to use the device elsewhere in your home where the wireless you usually use is too weak but the other is stronger then tell it to use the other wifi and forget the other. You can use the same password or key among both and the devices will not auto discover and then run with password or passcode from the other network.

[camerongray]

If you haven't already, you'd be best to set all access points to use the same SSID and security settings, your machine should then always connect to the strongest signal that it sees. You may also want to set the APs to use different channels from each other.  Also, as the second device is technically a router and not an access point, make sure you disable DHCP on the Asus.

[stuart85]

Keep 1 router on the upstairs and another on downstairs, connect both routers. watch this video to know how to connect two routers https://www.youtube.com/watch?v=a2wN4wJ2uMw you can also reset the router to its default settings so that old changes can be auto reset. click here to check the default id and password list.

[Allan]

Feel free to ignore the post from stuart85

[Geek-9pm]

Fo' r what it is worth.... 
A few years ago home network stuff was very expensive. and was not available locally.

That has changed and alters the why one can fix a home network problem.
Local stores like Walmart now have a wide assortment of routers and repeaters over a wide price range. Some below $20 hat work  almost as good as some selling for $80.

You might consider getting two or three and doing some experiments. Then if they do not work,take them back. Locals stores have a liberal return policy.

Here is  one style have tried and it has worked for over a year:
Wall Wort Wireless at Walmart.
This kind of thing is also sold on New egg and eBay. Often below $20 plus shipping. There is a bit of a learning curve understanding how they worth, but when you get it  it really does work. But not all routers will allow this mode of operation. So you may have to tinker with routers settings to let bridging mode work.
Just a suggestion...