>It worked correctly prior to this month.
Check out Organisation/Firmware Updates - and see if any firmware updates are listed as having happened around the time. If so, you might still have a "rollback" round arrow you can click on to roll the change back.
Check out Organisation/Change Log around that time. Any interesting WiFi changes happen?
Anything interesting in the wireless event log in Meraki?
Anything interresting in wireless health?
Also I'm assuming non-Apple devices are working ok?
Also I'm assuming you have only Meraki APs.