It looks like it isn't possible to plug the M4 into the phone line, so guess I need a modem/router. Is there a TP-link modem router which I can use as part of the mesh system, rather than plugging the main M4 into the router and using in Access point mode?

  @Omochao we're fibre to cabinet. As my current deal has come to an end, I'm going to be changing provider. I just wondered if it's worth ditching the use of the ISP provided router, as some people think you get more out of a third party one.

  @Callenbll 

If you are changing provider, my recommendation would be to use their ISP Router with Deco M4 in AP mode. If you need ISP Tech Support to troubleshoot Internet connectivity issues, it will be much easier to deal with them (with Tech Support) if you have their ISP Router in its default configuration.

Run Deco mesh in Router mode and provider Tech Support could dismiss your escalations as "third party router - not our issues." 

From my personal experience: I had quite elusive issue with frequent brief Internet interruptions. Having ISP Router in its default configuration helped to pinpoint where the issue is, which were outside of my home network. At one point I just powered off Deco mesh completely to ensure only ISP gear is in play. I can do that, because I run Deco mesh in AP mode and also my network diagram is similar to the following:

  @Callenbll 
If you are running a fiber optic, there must be BTU/ONT before the ISP router , kindly get the credentials on connecting to the internet from your ISP.

Check with them are their internet services using PPPoE, L2TP, PPTP or DS-Lite? and check with them for the VLAN ID details as well.

Deco routers far exceeds ISP routers which will usually be firmware locked, and not much of settings that you can play around with.