Normally Access Points are connected via LAN to Main Router and then provide Wireless Access from where they are placed , so in your situation if you had a LAN cable to garage then plugged it into WA801 it would give you wireless in garage or if you have a switch plug LAN into switch first the plug WA801 into switch so you have LAN and Wireless AP in garage.

If you use it as a Repeater then it will connect wirelessly to Router in house and repeat the signal into garage and maybe give you LAN access from the port in the back
( this is dependant on the WA801 if it supports that ) which the switch can be plugged into as well.

Remember to make sure your Main Router doesn't have AUTO set for wireless channel if doing Repeater Mode, it must be fixed to a channel or as soon as it changes the WA801 will lose it.

The disadvantage to Repeater is you only have so much Bandwidth on your wireless on Main Router and when using Repeater Mode it has to half the throughput to devices
as some bandwidth has to be allocated to being a Repeater the rest for Wireless Access.

That's my take on it , I have 2 x AP500's connected via LAN to my Main Router ( VR900 ) and these give me Wireless Access upstairs and in front room , I have flint walls :(
front room has a Switch first before AP500 so I can connect TV / Set Top Box / X Box / Media Box etc via LAN to save wireless bottle neck when streaming etc on TV ,
I did used to run a repeater in garage outside , but the wireless throughput did suffer a bit , now the VR900 is in Rear of house I can get this from garage so no need for Repeater.

However my AP500's do allow them to be used as Wireless Access Points as well !! or does this really mean Wireless Repeaters , it is something that is not clear and many manufacturers have called Access Points , Wireless Repeaters and visa versa , so beware when googling this some call it one way , others a different way and some really aren't sure.

Another note when setting up Networks TURN ALL DEVICES OFF then connect one at a time , you may need to allocate a fixed IP address for your Access Point in the Router
so it doesn't try giving its IP to another device via DHCP , you can never do too many restarts .

I would setup WA801 to Router give it a fixed IP then restart both ( eg : If router is 192.168.0.1 then make WA801 192.168.0.2 ), see if you have wireless Access on WA801 , if yes , take it to garage and see if you still have Wireless Access , if yes , connect switch , wait , restart router / WA801 / switch then when settled try a PC via LAN Switch on WA801.

Removing DSL cable may be sensible till you have it all working or BT's DSLAM god will get pissed off and may well think the disconnects are a faulty line and push your Interleave up
for a bit , it will recover but can take a few weeks , so setup Network first then reconnect DSL line when done, if you need to see if you are connecting just try signing into router admin
page , if you can you have access .