You could configure "Concentrator based Layer 3 Roaming". Refer to this section in this document:
https://documentation.meraki.com/MR/Client_Addressing_and_Bridging/Layer_3_Roaming
One trick I have done in the past in this case - which is super simple - is to create a network specifically for wireless, and put all your APs in this one network - even though it spans multiple sites. Then when you get authorised on on AP in one site and go to another site Meraki still things you are authorised because it is in the same network.
One downside (it was an upside for me) is that you can only have one splash page.