What fixed it for me was resetting configuration (System → Reset Configuration and check “No Default Configuration” box).
Connect to the router via cable then use Winbox to access router via MAC address.
When setting WISP AP (via Quick Set interface), set “Mode” to “Bridge”, set “Address Acquisition” to “Static”, give it an address in the same subnet the DHCP server range and DO NOT set “Gateway” or “DNS Servers” parameters.