I've seen this a few times, where a dual-band single SSID may cause initial setup issues with IoT devices...
What's always worked well for me is to setup a single band SSID short term, so that the Device and Mobile App can do the initial pairing...
Once everything is hooked up, one can disable the temporary SSID, and things should be fine
Once going, one should be ok with g/n connectivity setup for 2.4 - never set it to 802.11b mode only, as this will suck the airtime out of that radio...
Keep in mind that 11g/11a/11n/11ac/11ax/11be are all OFDM - and as such, the basic rates are the same - 6Mbps is the baseline for all OFDM targets...
I'm also not a big fan of setting the radio to a single mode only - the firmware is smart enough to sort it out on it's own - in 2.4, g/n generally works, in 5GHz let it float on Auto, as this covers 11a/11n/11ac/11ax/11be...