You say that there are a few routers in your network. That "complicates" the situation as I need to know more about network topology before being able to post a right answer. What computers connect to what router, what router connect to what router etc. I don't need specific information like "comp a connects to router boo" and repeat the operation for each computer. I need a general information, something like 10 computers connect to router X through ethernet, 20 computers connect to router Y wireless, network IP addresses. Also, important are the switches. The switches that allow management could are very-very important to mention.
BUT this is SENSITIVE information, pay attention what you say to whom.
As far as I can assume, there are 2 cases:
case 1) The physical network is divided into a few small networks, located in different locations. Different buildings in town or something like that, not your "personal" cables between them (companie's), so you have to use internet to connect to those "network branches". In this case... VPN is the sollution. I can't help you here with anything but naming the VPN.
Or a ftp server. Eventually with port forwarding at your routers.
case 2) The physical network is in a single area, a building or a few buildings nearby, or in the same yard. Where you use some of "home" routers for connectivity. Depending on your particular situation you may need to use port forwarding or solve the problem without port forwarding. But I need sensitive information: at least the network map, which router connects to which router, model of routers, if you have switches, switches with management, access points, dhcp servers, dhcp pools etc
In the image, you hid the ip. It's a good measure of security. Unfortunately... For your problem to be solved, you need to provide me with more information which is (I repeat) SENSITIVE INFORMATION for your company: network map, IP's, devices roles in the network (desired device roles in the network in fact).