Chime Pro II not connecting over 2.4GHz, only 5GHz

This problem has been an issue for months now, I'd worked round it by blocking the 2.4GHz mac addresses for the devices from the Wifi network, so they can only connect at 5GHz, which worked (after speaking to Ring tech support and them even sending replacement devices). I've left it as it worked, but now I'm looking at it again because i'm dropping the Security VLAN they connect to onto 2.4GHz only (another story relating to camera connectivity you'll find on here as well!). I have used chime pro 2 devices for 3-4 years without any problems on 2.4GHz, then all of a sudden it stopped connecting. i use Ubiquiti AC-HD/AC-Mesh and AC6 Access points, connected to Ubiqtui PoE switches and an OpnSense Firewall. Nothing changed between the device working and not working, and after having forced to use 5GHz for about 6 months faultlessly, I've tried to get it working again - same problem. All other Chime II and Chime Pro 1st Gen devices are connecting perfectly on 2.4GHz - but not these. Can anyone help - anyone seen this issue? Also it seems NOT to be related to the Ubiquiti network - If I connect to my phone as a wifi Hotspot, it connects perfectly, but my iPhone defaults to 5GHz. if I enable "compatability mode" on my phone (which forces it to 2.4GHz) - I have the same problem! It seems a firmware issue with the Chime Pro II but Ring have not been able to help at all. From the Ubiquiti network, I can see the Chime Pro II come online on the 2.4GHz network, but not pick up an IP and drop off again.