Using 3rd party mesh system with EE Broadband

[Javert]

If I want to eventually use a 3rd party mesh system after getting EE broadband, do I just need to set aside the EE router and plug one of the mesh boxes direclty into the BT Openreach ONT/Modem?

Also  - does this mean that an landline I have specified with EE won't work so there is no point getting a landline number if I am going to do this?

[XRaySpeX]

Yes. EE does allow the use of 3rd-party routers. However they may only investigate any issues until you put the supplied EE router back. In Using & Configuring a Third Party Router you'll find everything you need to set up your router incl. the BB PPPoE creds. which are now the same as BT's.

The DV landline will not work with your 3rd-party router unless you put the EE router as 1st downstream from the ONT.