@user_jw21 

Go into the wireless settings, enable the AP isolation for 2.4GHz, save it, then disable it and save it again. 

You should still be able to see any device connected to any of the wireless bands.

@Tony 

Thanks! Just tried as you suggested. It's actually located under System Tools > System Parameters, but I found it. No luck though. Any other suggestions?

@user_jw21 @Tony   having the same issue - whenever SmartConnect is enabled, one or more of my speakers drop off the Sonos system and/or I can't get the app to find the entire system at all (intermittent). Further, I cannot get some speakers to reliably connect even with SmartConnect off unless I also turn off auto channel assignment for 2.4ghz and set the channel manually. I don't think the issue is the phone being on 5ghz because with SmartConnect off, the phone is still on 5ghz and Sonos functions exclusively on 2.4ghz and it works fine. 
 

Manual channel assignment with SmartConnect off fixes the issue, but I hope TP-Link will investigate because one of the reasons I paid so much for this router was the feature set; there is no reason why SmartConnect/band steering can't function with Sonos devices. My 7 yr old Netgear functioned better than this brand new model with Sonos :(

  @user_jw21 I can report as of March 2023 "Smart Connect" still breaks Sonos connectivity. I'm running an Archer AX21 v2.0 with firmware 2.1.5