@wbob 

I would probably try taking half or maybe 10 of the devices and moving them over to the guest 2.4G network (different SSID/password). Reboot the router and see if the devices are then able to connect. 

@Tony Thanks for the reply!  I have already decided to try your suggestion with a slightly different approach - also by reducing the device load on the processing power of the main router (MR).  I purchased a new TP-Link Archer A5 1200 for under $30.00 and, just yesterday, set it up as a wireless access point (AP) connected to the MR via two power line adapters.  I know that there are people that do not like these adapters, but I have been using them for years and I get bandwidth better than the wireless signal near the router - including enough to watch 8K TV (that's another fantasy).  The wireless AP is using a different SSID, password, and channel (to not overlap on the 2.4 GHZ band) than my MR.  This also has allowed me to identfy 12 smart switches/plugs that have the weakest radio signals and place the AP at a location that will now provide all 32 of my devices with a radio signal no less than -55 db.  Keeping the SSID/Password of the AP different than the MR also ensures that these 12 devices will not "roam" back to the MR.  Note: My home is 3,750 sq. ft. and three floors.  While I have measured the signal strength and determined the best location for the AP, I next need to "reset" the 12 smart switches/plugs with the new SSID/Password.  Besides the 12, there are other smart devices that can move from the MR to the AP and further ease the load on the MR.  I will have to experiment.

The added bonus is that I now have a great wireless signal throughout my house including the two areas that were deficient - a home theatre room (because of four rooms [walls] between the theatre room and MR as well as a distance of about 70 feet) and the garage.  A mesh sytem would probably provide better, or at less  more seamless, roaming than the AP.  However, the only roaming I do is with my iPhone and, so far, it seems to change routers with little problem.  Note: The home theatre room was my first successful utilization of power line adapters to the TV via  wired connection and speed tests show only a loss of 1 Mbps compared to the bandwidth I am paying my ISP for.