Not showing as available network

While you are waiting, you might try running a WiFi analyzer on your phone to identify what channels your router is broadcasting on.  I don’t think this is your problem because you say you can’t see 2.4 GHz bands, but it is known that some older devices don’t play well with WiFi 6 “DFS” channels (52-116 and 132-144).  So, it might be interesting to note what channels are in-play when you can’t connect to these devices.  Unfortunately, the router auto-selects different channels at random times, and if it happens to decide on DFS channels, and you have older devices that pre-date DFS, it can create problems with connections (I have this problem with 2 older android tablets).