I found the root cause, and it's very silly!
What happened was that the LAN/Ethernet cable was connected to the modem, not the router.
When called the ISP customer service:
1st Representative (8th June): Asked to connect the phone line to the TEL1 port, disregarding the DSL port
2nd Representative (8th June): Asked to connect the LAN/Ethernet cable from modem to computer*
3rd Representative (today, 10th June): Said connecting to modem is incorrect, should be connected to the router -> connect to router -> issue resolved
*Point after when I started this thread
Thanks for the help and for being patient Geek-9pm, this thread can be closed.