WiFi extender help needed

Hi guys,

I need some advice from somebody that knows a little more about wifi than I do. My new house is a single story and has a garage that is about 100' from the main wifi hub with two concrete walls between. I am running a Asus RT-AC66U main router and have a RT-N12 router running in extender mode. The wifi stability is not great and I am only getting about 1mbps throughput on the extension router.

Now I have some old CAT5 between the main router and the extension router but I don't know if the cable is any good. It has been cut in two places with ice block ends and RG45 splice blocks so I don't trust it. I may be able to run a new CAT5e cable but it will be another attic crawl I would like to avoid.

I read somewhere that your extender router should be the same as the main router but I am not too excited about dumping another $200 for a second RT-AC66U but I might do that instead of the attic crawl if it can get my speeds up.

I previously had seperate networks but my Sonos system will only work if every controller is on the same network.

Suggestions?