How to use a router as an extender when it works from channels 1 to 11 when the main router only switches between channels 6 and 13?

I was using an old router (TP-Link) to use it as an wireless extender to my main router. The problem is that the main router channel is in auto, so that it provides the best signal and it mostly switches between channels 6 and 13. Now, my TP-Link router has only option of channel selection from 1 to 11. So whenever the main router goes to 13, the TP-Link no longer can scan that WiFi for extending its signal and will be without WiFi. I have downloaded the latest firmware and still I couldn't find a solution. Some suggest change region of router, but I don't find that option anymore.

Question by Kushal Khatiwada on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.