The router is not working correctly, how to fix it?

Question

network hardware

Hello, I have a TP-LINK TL-WR840N model, with a strange problem, the answer to which I did not find on the network
. Namely, the router refuses to enter the settings page. ( tplinkwifi.net or ip)
I currently use a 4G modem and distribute the Internet through a router, what to say about its performance.
It appeared that it was necessary to accept the Internet on the router ... In general, with any manipulations, I get the following

Oops! It looks like you aren't connected to your TP-Link network.Please check your network connection and try again.
Visit official TP Link website >

This option if a 4g modem is connected
. If I turn it off, the page does not load (which is logical)
I hope someone came across and solved this issue.
Thanks

Answer 1

Reset to factory settings and try to go to his IP?

Answer 2

Or else there is an option, flash it on DD-WRT and NEVER again suffer with firmware from TP-LINK) I flashed my TL-WR1043ND and gave it to a friend, it became a problem-free router!