If it is connected with a cable, ensure the port is the WAN port on the sprinter, and on your main router, in Guest Segment, ensure under Ports and VLAN Settings, that T is on the port where the extender cable is attached. Show other segments should have a tick for Home Segment on the same port.
Under IP Settings, make sure Use NAT is enabled and the Internet Traffic Handling Rules "Connection Policy" has your provider ISP connection selected.
If the Guest Wifi works on the main router, personally, I would remove the extender from the mesh, factory reset and rejoin. Then you will know if it is a config on the main router or the extender.