AMP and SUB connect normally connect via a private 5GHz link. 5GHz is easily blocked by walls and distance. BOOST is normally not used in this sort of configuration. However, if you promise not to tell anyone that I suggested this alternate plan, here is something to try (no guarantees): Place a wired or wireless BOOST or any player with a network port between the AMP or another wired player. Wire the SUB to BOOST. While it is recommended that isolated wireless players not be wired together, this will often work, maybe with an occasional glitch. In spite of the recommendation to not do this, I used this trick for a couple years and I only had a couple glitches. Reboot worked through the glitches. I’m assuming that AMP is wired to your network.
 

If your garage is on the same electrical panel and the A/V closet and garage breakers are in the same column in the panel, EoP (Ethernet over Powerline) might work. I have very low expectations for this configuration, but there can be surprises.

The sonos amp is actually connected to my wireless wifi network, would this change your recommendation?

If not you’re saying purchase a boost and connect the sub to boost with ethernet connection then this will allow the amp to connect to sub via the boost signal? 

Connectivity to SUB is the issue. Do you have any other SONOS units? BOOST is simply a player with the audio stripped out to reduce costs. You can try my scheme by temporarily using another player.

Yes I do but the sonos unit I am trying to pair with the sub is the closest sonos unit to the sub location. Maybe the sonos amp in my basement for the patio speakers is closer now that I think of it. If so how would I connect the boost to the system?

BOOST could be wireless and wired to the SUB. As a temporary lash-up you could use the AMP that is not Bonded to SUB located where you might want a BOOST. Make sure that the Bonded AMP is wired to the network.