Have you changed the connection to the main router? Not using repeater connection anymore? Or repeat the 2.4GHz WiFi?
If repeat 5GHz WiFi, then it should show the channel value of the parent router here, so you can’t change it.
If you are repeat 2.4GHz WiFi, the speed will be much slower.
If you have switched to a network cable to connect to the main route. Then you can check which channel is not being used and set it to that channel to get the best speed. NetSpot can help you check channels usage.