When you change to Access Point network mode, there should be a message like:

If so, you may not be able to assign the GL-B1300 sub-router with a static IP address that is accessible after the router reboots. The router will automatically get a DHCP IP address from the main router, so log into the main router and look for the client sub-router’s MAC address and its current assigned IP address. You should connect the sub-router WAN port to the main router (I think WAN and LAN ports are bridged, but not sure).