Try to keep your MPLS VPN up/connected is the best advice. Your APs will lose
heartbeat connectivity with the ZD and eventually reboot after the Auto-Recovery
period of time you may have set on Configure/APs page, AP Policies.
More importantly, your remote clients will not be able to use any back-end auth
methods that involve the ZD, AAA servers, HotSpot, WebAuth, or communicate
over any Tunneled WLANs, however your clients using any L2 type of connection,
like WPA2-PSK will still have access to local resources (servers/printers), until the
APs reboot. Clients connected by the other methods will have local resource
access, until they roam (and cannot re-authenticate).