@SayeemAhmed Thanks for the response but not sure how that helps. Connect the cable to what? I need the WiFi mesh extended.

  @Bandyrobs You can connect more Deco units to extend the range.

Bandyrobs wrote

I have been using our DECO M4 mesh (3 devices) in a previous house for a few years without issue. Have just moved to a 300 year old stone cottage with thick walls. Generally the mesh still works well but struggles to reach one particular room. 

 

Have tried to use a belkin AC500 but this doesn't seem to help.

  @Bandyrobs 

For house with thick walls that do not allow WiFi signal through using powerline adapters is an option. Which you tried. Can you explain in more detail what went wrong or did not work with Belkin powerline adapters: they can't establish link to each other over electrical wiring or it were something else that didn't work?

Also, while this is an old house, someone might have layed coaxial TV cable through it, and if they did - that cable can be used, with proper adapters, to pass Ethernet signal. If you have TV cable going through the house, I can discuss that option in more detail.

  @ARGamingPK1 Thanks. That was something I was considering. Do you know if you can mix and match nodes/units. I.e 3 x M4 and 1 x X50 ?

  @Bandyrobs 

You need wired link to make it work, if powerline is not stable and coax/Ethernet is not an option, you may be out of luck.

You can mix X50s and M4s, but that will not improve anything. Newer versions of WiFi protocol are faster but more sensitive to obstacles. M4 is WiFi5, it'll send better signal through the walls than X50 which is WiFi6. Saying it other way, X50 might be worse than M4 in your situation.