Running the garage router in WAN to LAN mode is NOT running as an access point. You want LAN to LAN mode. Right now you have things set up where your internal network appears as a WAN to the garage network which is not correct.
Set up the garage router in LAN to LAN mode, connect it to your normal router on a LAN port. Disable all the DHCP server, etc. in the garage router.
Then you will be good.