Did cell performance actually improve when you turned off Wi-Fi? Or did you just see more bars?
Try turning off Wi-Fi on the phones.
He didn't mention the phone or the carrier - but ATT and Verizon have enabled WiFi calling on certain handsets (mostly higher end android and iphone 6s and later that are VoLTE capable) - so the RSSI bars and poor performance when WiFi is enabled could be due to the WiFi calling feature...
WiFi calling can be very useful, but it is dependent on the Router/AP/Gateway - when the stars align, it's very handy to have...
On iPhone - to see if VoLTE is enabled... Why VoLTE? Because WiFi Calling uses the same Carrier Gateway as VoLTE does, and as such, allows handover from LAN to WAN during a VoWiFI to VoLTE handoff.
Settings -> Cellular -> Cellular Data Options -> Roaming -> Enable LTE -> Voice & Data (to disable VoLTE, set it to Data Only) <-- VoLTE is generally a good thing to have in most markets
to check for WiFi calling
Settings -> Phone -> Wi-Fi Calling -> Wi-Fi Calling on this iPhone (on/off) <-- try either way, WoWiFI can be nice, but if you've got good bars, then consider turning this off if you have performance issues.
On ATT iPhone - to double check...
Settings -> General -> About -- Scroll down to Carrier - tap on carrier, and if it changes to IMS Status == Voice & SMS, then VoLTE is active on that device...
Some Router/GW/Firewalls might have issues with VoWiFI and SIP/RTP handling*, and multiple Router/AP's repurposed as AP's can contribute to the problem if not setup correctly...
*SIP ALG settings, which are not consistent from vendor to vendor, and sometimes even between models/firmware revisions from the same vendor.