It may not be possible if the ISP has loaded custom firmware on it. You could try to get other firmware but I bet they locked the firmware update option or removed it entirely.
In general it shouldn't really cause you that much issue. All it is going to do is scan for the area with the least neighbors using wifi. That is what you likely would do if you set it manually. It really doesn't matter there is no good channel to select anymore all tend to be in use.
The one key advantage to letting it run auto is it can select channels that are more restricted. They are used for weather radar and the device must change channels if it detects radar...which is many times only on during storms. Many routers do not support this...