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 because of the complexity but you never allowed to set those channels manually anyway.
I would try it and see how poorly it works. Your only option is to disable the wifi in the router and buy another device to act as a AP where you can control the wifi radios.
The only issue I'm facing is that the upgraded speed i.e 100mbps works only on 5GHZ with 80mhz ; some of my devices like Amazon fire stick & few old mobiles are unable to connect to the network with these parameters.
Contacted Amazon CC, & according to them Fire stick catches on to WIFI till channel 45 only, hence need to be able to change the channel !
That is a strange restriction. 45 technically is not a valid channel but the 80mhz lower block is 36-48 so maybe they just call it that because it is the middle.
If firestick have that restriction it is likely they are being lazy and trying to ship only 1 product for the whole world. Most countries allow for the blocks above 149 and many also support the channels between that have the weather radar type of restrictions. There are countries that do not allow these so maybe they want to not have different versions.
Your problem is you now have 2 devices that are problematic. One that wants certain channel and the other that you can not assign fixed channels. If you only had 1 of these issues you could solve it.
Your only option is to get rid of one of the devices. If there is a better tv stick you could look into that or you could add a AP/router in front of the ISP router so you can set the channels.