@bazzason 

I would suggest to add another unit. 

You can also try powerline adapters to create wired link. If you are interested in this option, you can test its feasibility with X50s you already have. Here is how:

1. Get pair of powerline adapters from TP-Link brand, I recommend TL-PA7017P KIT or TL-PA7017 KIT. 

2. Place one of powerline adapters in the garage and one in the house, on the same circuit breaker and near either Main or Satellite Deco.

3. Verify that powerline adapters established link to each other.

4. Borrow one of in the house Satellite Deco, bring it to garage.

5. Connect with Ethernet cable in house powerline adapter to in house Deco unit.

6. Connect with Ethernet cable in garage powerline adapter to in garage Deco unit.

In garage Deco unit should report Ethernet backhaul. 

You can now test in garage WiFi speeds. I won't promise you much, but getting 100Mbps is quite possible.

If all that works, buy another Deco unit and I would recommend X50-poe for consistency. If this does not work, return powerline adapters to seller (get them from retailer with return policy).

  @Alexandre. 
 

Thank you 🙂 I will try this.

If I'm understanding you correctly, this will only work as long as the in-house adapter is plugged in to a socket on the same circuit as the adapter plugged into the garage socket.

Does distance affect signal strength? For example, will plugging into a house socket further away from the garage degrade the signal strength? We are talking less than 40m of T&E cable between socket to socket.

  @bazzason 

With powerline adapters it is hard to tell what speeds you'll get until you try. It depends on quality of electrical wiring and what else is plugged to sockets on same circuit breaker. Anything with motor, such as fridge, also microwave and dimming lights are powerline speed killers. 

So, just give it a try.