I apologize, but I just read the initial post more carefully. Am I reading it correctly that your apartment and garage are on the same wired network? If so, this is less about mesh networking and more about a consolidated SSID. Wireless mesh is more referring to the backhaul used to connect the access points to one another, where this is done wirelessly rather than wired.

If, however, you have CAT6 running to that second location, then you don't need a wireless mesh system, you just need a system that hands off between devices gracefully. Unifi is fantastic for this type of setup.

Without knowing more about the layout of your apartment and garage, I'm assuming you have a single-floor apartment somewhere in the 500-1500sqft range, and a typical 1 or 2 car garage that's somewhere between 120-200 sqft? In that case I'd highly recommend a Unifi Dream Machine for the apartment and a Unifi AP-AC-IW for the garage (there's a Pro version but it's more than twice as expensive and only a little faster which I assume you don't need in the garage). It's a nifty access point that goes over a typical low voltage box and replaces the face plate with a small playing card-sized AP. It even has a built in switch you can connect other devices to.

To power the in-wall AP you'll either need a POE injector between the Dream Machine and the line going to the AP, or you can add the Unifi Flex (not Flex Mini - both can be powered by POE but the former is able to pass POE to the other four ports or provide it its self) for an inexpensive 5-port POE switch, which would expand the ports on your UDM anyway. Or get a Unifi POE switch in a wide number of configurations depending on your needs. They have 5/8/16/24/48 port switches in both regular and POE varieties, some with fiber uplinks