I set up my wired D-Link modem in windows easily, but in ubuntu, it is a bloody job!

I want to set up my wired ADSL2+ D-Link router. It is simply:

1. Typing 192.168.1.1 in address bar.
2. username and password
3. In Ubuntu what appears is: It needs clicking refresh bottom multiple times for the next window to appear:
4. During set up through Advanced Setup menu, by pushing the botton Next in each step, the error above, namely 400 Bad Request appears multiple times. Again I have to click refresh in order to go to the next step. It may take half an hour to reach the final step which re-starts the modem automatically.
   This all is in obvious contrast with what I experience in Windows 10. In windows, set up can be done easily and cleanly without any 400 Bad Request error.

Question
What exactly the problem is?

OS: Ubuntu 18.04 , Windows 10
Modem: D-Link DSL-2520U

Edit
The output of lspci -knn | grep Net -A3; uname -a is

08:00.0 Network controller [0280]: Ralink corp. RT3090 Wireless 802.11n 1T/1R PCIe [1814:3090]
    Subsystem: AzureWave RT3090 Wireless 802.11n 1T/1R PCIe [1a3b:2041]
    Kernel driver in use: rt2800pci
    Kernel modules: rt2800pci
Linux soha 4.15.0-96-generic #97-Ubuntu SMP Wed Apr 1 03:25:46 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

Why did I re-set the router?
Actually, I had no network whereas everything was seemed fine; I mean, when I was trying to go to a web page, this was appeared in that tab:

The gray circle which rotates counter clockwise for up to 3 minutes; Then it depended on chance; the page might finally open or not. I called to the provider and they said "we see no problem in our side"; The modem also showed no indication of a problem; DSL and internet lights were ON. As I saw no solution, I re-set the modem.