contact support and ask then to enable 2.4ghz only support on your network.
and try set the minimum bitrate to 1
I have never been able to get my FitBit Aria to work with a Meraki AP no matter what. I have spent hours fighting with this and all I do is get angry and start saying bad words.
Though, I have not tried asking support to set 2.4 only... But I have tried dual band operation and minimum bitrate to 1.
I have tried every b bitrate, and set all SSID's on the AP to dual band operation, with no luck. It sounds like the OP has done the same.