I think only BT and Sky do self-install so far. I'd be surprised if they don't offer the data extension cable any more, (which would cause an engineer install), but perhaps they don't.
You have two or three other possible solutions.
Plusnet do engineer install by default, and the free data extension cable (Home Wiring Solution) installation is an option in the order process. It would be a modem and separate router.
If you put CAT 5 in place beforehand, with a bit of spare at each end, then that is as good as the data extension cable and the engineer should be happy to connect it up. You would still need to order the data extension cable, to make sure the wiring job is on the task sheet. It depends who you think will do a neater cabling job, you or a professional. I'd choose the engineer.
That would put both the modem and router where the extension is now.
The second way, if you are happy with it, is to have a bog standard installation, with the modem where the master is. It's the modem position that matters, not the router. Then run an Ethernet cable using CAT 5 up to where the extension is and put an Ethernet socket there for the router. Best practice is also to put an Ethernet socket near the modem and use a short patch lead between the two.
This second method could also be used on a self-install from Sky, BT or whichever, where usually a modem/router is supplied. You would need to buy an HG612 Openreach modem off eBay.
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