An easily avoided stumbling block, is that the router location in the main building, and the external garage, are in different ring mains. Quite possible.

In reply to a post by heathmount:

"Out of interest what is plugged into the socket on this picture?"
An RJ11 cable into a second router

You do not use the RJ11 input for the second router that should be *configured as an WiFi Access Point* .
you use one of the RJ45 sockets for the input from the main router.

* = you can see plenty of internet posts how to re-configure the second router.

In reply to a post by APTMAN:

In reply to a post by heathmount:

"Out of interest what is plugged into the socket on this picture?"
An RJ11 cable into a second router

You do not use the RJ11 input for the second router that should be *configured as an WiFi Access Point* .
you use one of the RJ45 sockets for the input from the main router.

* = you can see plenty of internet posts how to re-configure the second router.

You make a good point but it did cross my mind that whoever put the RJ11 socket in should have explained to the OP's friend where the cable (on the back of the socket) goes too as it currently seems to be a mystery.

Edited by deleted (Sat 23-Apr-22 23:29:07)

In reply to a post by dect:

You make a good point but it did cross my mind that whoever put the RJ11 socket in should have explained to the OP's friend where the cable (on the back of the socket) goes too as it currently seems to be a mystery.

To me eyes it looks like an RJ11 plug in an RJ45 socket. Perhaps easier to see if you magnify the picture.

OP is discounting (for now at least) the existing link:

“Sounds like "something" has been run into the garage but not something that will work. I'll be round there this weekend to sort out the powerline method.”

To be fair he’s right there: without checking what grade of cable has been installed and to where, then it’s fairly useless as a viable data link, irrespective of the jack type on the faceplate.

Here is what I would do..

* remove that Huawei 8010 ONT from its current position.
* install a fibre terminal box in place of the ONT
* [replace blue SC-UPC connectors with green SC-APC equiv.]
* extend incoming SC-APC fibre, with looooong APC/APC fibre patch lead
* connect APC patch lead to newly-sited Huawei ONT, and onwards via ethernet to home network
[or better, keep it all fibre with ONU and OLT SFP sticks in dual-SFP+ PCIe NIC in OpenWRT x86 box]

Cost [excluding x86 router box] less than £50.

Very tidy future-proof full-fibre solution.

https://imgur.com/a/kdDpyw5

Seems quite a lot of messing about to relocate the ONT (and router) when all they only wanted was a network extension to the garage…😎

Not to mention making Openreach unhappy that you've messed with their end of the network.

Probably easier and safer to run some Cat6a