What is Reddit's opinion of Tupavco TP542 Dual-Band (13dBi) Outdoor Directional Panel Antenna (2.4GHz & 5GHz WiFi) Wireless Network Signal (Pole Mast Mount) Weatherproof High-Gain Long-Range (w/N-Female Connector)?

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Can’t vouch for this specific product, but a weatherized AP in the garage with wire back to the house or a directional antenna from the house to the garage would be your best bet. You need WiFi for updates, plus you may find having outdoor coverage leads to using the space more.

https://www.amazon.com/Tupavco-TP542-Panel-WiFi-Antenna/dp/B015QF7EMK

Agreed, likely a directional outdoor wifi antenna for boosting signal over on that part of the building . Quick search it looks like this

Maybe swap one of the router antennas out for a directional antenna and point it to the other location? Something like this for example:

https://www.amazon.com/Tupavco-TP542-Panel-WiFi-Antenna/dp/B015QF7EMK/ref=mp_s_a_1_5?keywords=directional+wifi+antenna&qid=1576066633&sprefix=directional+&sr=8-5

15cm^3 is a pretty tight constraint. I'd probably go with some kind of panel antenna like this for something dialed in around 2.4ghz.

https://www.amazon.com/Tupavco-TP542-Panel-WiFi-Antenna/dp/B015QF7EMK

Get routers/aps with replaceable antennas and get directional antennas. Like such: https://www.amazon.com/Tupavco-TP542-Panel-WiFi-Antenna/dp/B015QF7EMK