Configuring router as client to share internet connection

I have a task that involves setting up openWRT on a Netgear r7800 router and some ubiquity access points, and setting up one vlan for guest wifi and one vlan for admin access. The netgear router has no direct internet access, so I tried configuring it as wifi client that connects to my internet router and allows internet access for all clients.
It used to work before, but I had so reset several times and now I can't make it work. The openWRT network page of the router shows 2 wifi radios. One is successfully connected as client to my Fritzbox internet router. What do I have to do to so I have internet connectivity for the clients connected to the Netgear router?
I tried changing the firewall zone and bridging the interfaces, which mostly resulted in having to reset the router again.
Any ideas?