You are correct. I confused the Smart Device Network page as being similar to Smart Steering.
Try turning off the two 5GHz radios when setting up the 2.4 GHz devices, to see if that helps.
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Just wanted to share that It's been about 5 months since I last posted and I've been running fine with my T3200, 6 Wifi Boosters and many Apple (and smart home) devices including XR and MacBook Pro. Not sure what the final fix was (might have been a complete factory reset of my Telus devices), Smart steering off, hidden separate SSID 2.4 network for my older 2.4 only devices from the T3200 and everything else running off the boosters on another SSID.
Funny enough - I read about the idea of shutting off the 5G bands this morning and decided to give it a try. It takes a bit of fighting with the web interface to get both 5G Bands offline, but thus far it seems to work. I'm slowly going around my house and reconnecting my smart devices to the Wifi with the 5G off. Oddly, the cheap devices haven't been giving me much trouble....the Wemo ones (which you'd expect to be more polished?) aren't fairing as well.... oh well. I'm at least hopeful at this point that I'll get everything up and running soon!