I finally seemed to solve the issue.
I did a complete factory reset, followed by converting wan port to separate lan interface for 4G connection at 192.168.234.x subnet. For the dumb ap switch part, I set the original lan interface for eth0 for the correct static address in the intended shared subnet, and added gateway and dns addresses to /etc/config/network. I also enabled lan bridge only to eth0.
Despite providing wireless ssid for network “lan”, I couldn’t get the wifi to provide ip address to client from the network dhcp server. I created another test ssid with correct settings, disabled the correct one, rebooted, disabled the test ssid and enabled the correct ssid. After that I could connect to the dumb access point.
I hope this helps someone else 