Go through the links I sent and see if that heps. If you still have that issue, try to scan again and see if the best channel for you has changed - you can change the channel according to the reslut. If you live in highly congested area it might be better to think for upgrading to a dual band router and avoid such kind of issues.

@Ivaylo can you suggest me some good router

It really depends on your usecase. Judging by the use of a range extender and a second router (in AP mode probably) I presume you need a better Wi-Fi coverage. If you decide to stay with the brand and want to be in pace with the technology I would recommend one of the AX entry level series which is OneMesh capable (I believe AX20 for now, AX10 is in development). But if you prefer a budget solution you can go with something like Archer C6/A6. Of course you have to read in details (reviews, customers' opinions, this forum) about the chosen model to be sure that it's the right for you. If I were you, I would wait until after next year comes to go on the market.

@Ivaylo thanks a lot