Auto reconnecting wifi when my iPhone SE2 locked

I noticed that my phone auto disconnect from wifi and then connect to my other router sometimes it re join to the same router when phone is locked. I tried turn off both routers auto re join option but then It didn't even connect to the same router again. And tried almost every possible solutions on websites but nothing works out. What do I do? I can just turn off other router's auto rejoin but I think that's not necessary? or is that the problem? (both router's rejoin options are enabled)