@vengenerator
Hey
In short TP Link have 2x Mesh ranges, these being
One Mesh - Works on some routers, APs, Homeplugs and extenders
Deco - All deco devices
Sadly these 2x mesh setups are not compatible
namely OneMesh wont "mesh" with Deco if that makes sense.. Sadly now you are down the deco route you are better staying on it. Having used both, generally imho the Deco is the better route.
As for a router... I also have Plusnet and use a VR600 with the WiFi disabled, you will likely have to do the same so I would therefore recommend just getting the cheapest "reasonable" router (something like VR600) and the money you save on that put towards a new extra M5 unit sometime.
Option2 .. if your contract is near up with Plusnet, call and renew for another 18m and they will give you a new hub for free. Outside that its £99 so don't waste your money buying it
Option3.. people are always selling Plusnet hubs NEW on ebay as they already have a router. These are around £10-15 on average and literally NEW.. just log in and change the username/password good to go. Again what you save add to a new M5 . One here 2 days left NEW for £8 at time of writing
https://www.ebay.co.uk/itm/Plusnet-Hub-One-Wireless-Router-New-Sealed/284136489924?hash=item4227dac3c4:g:jOEAAOSwWu1f35Ky