The only device I've ever had significant problems with regarding WiFi in the past 5 years was my GMG grill. I used to use an AirPort Extreme dual band with both having the same SSID and now have an Orbi mesh system and a number of devices have existed under both systems and never had a single problem. This includes a few older devices that are 2.4 GHz only.
My understanding of most of the mesh systems is the 2.4 GHz only devices should be able to connect to and keep a 2.4GHz only connection and devices that are capable of both can connect to either or both channels for better bandwidth if they support simultaneous dual band. The implementation and compatibility of this could obviously vary from brand to brand of mesh routers and even from one firmware update to the next.
Another thing that shows up VERY frequently when people are having the 2.4 GHz problem is that the problem device is often being setup and connected to WiFI from another device like their phone or tablet. The problem here is that if the phone supports 5GHz (what smart phone that people would still use today doesn't?), it's going to connect to that. To get around this, you might be able to forget your WiFi on your phone, turn off 5 GHZ on your router, connect the phone to 2.4GHz and then to the device to configure via bluetooth and set it up. Then you should be able to turn the 5GHz back on and repeat the forget and reconnect process on your phone to get it back on 5GHz. This would all be dependent upon whether the device you are connecting is getting network information passed over from your phone (like BSSID) or if it really is building a connection inside it's application that you are just controlling and entering data into via the bluetooth connection.
I haven't had the GMG for at least 2-3 years now, so I've forgotten a lot of the details around it's Wifi issues and how it's setup, etc. I just remember buying a 8-10 inch antenna, a cheap Android pre-paid phone, getting a new controller from GMG after troubleshooting, etc. Maybe the thought above pertains? It seems like you connect to the P2P WiFi on the GMG rather than bluetooth though, but do you give it all the connection information via the app when connected or does it pass it from your phone where it could be sending it the 5GHz BSSID or something? Can you turn off 5GHz long enough to see if you can connect fine when it's off?
