I also contacted support. They replied to say "it is recommended to use IPv4 only". This is not particularly helpful so I will be returning the device.

@apt If you go to Advanced > Network > Internet and then select "Create Profile". Copy all the same details from the default EE profile, but change "Profile Name" to "EE2" and "PDP Type" to "IPv4 &...

@apt The MR600 is supposed to support IPv6, so if you enabled IPv6 then you should be able to avoid CGNAT because CGNAT is only used by EE for IPv4. You could connect from external devices to your...

@AlfieFogg Did you ever manage to resolve this? I have the exact same problem as you, which is that when set to both IPv4 & IPv6, the router only seems to obtain an IPv6 address and so IPv4 services...