Hoping someone can point out the obvious thing Im apparently missing here!

I recently replaced a fritzbox 7490 with a Spark Smart Modem 2 that I had left over from a business.

The SM2 is all up and running fine and connects directly to the fibre in the tv room.
The Fritzbox Im trying to re setup out in the garage to connect to the WiFi of the SM2 but no matter what I change it will see the main WiFi network but its always greyed out as not being an option to select.

Makes me think its a security protocol or similar not allowing it.

This Fritzbox is to connect to the main WiFi to run just as an access point which has a little 5port POE switch with 2 cameras attached to it
Doesnt need to broadcast WiFi at all