I come with an update.
So, if you are using the "3internet" APN and have the PDP type of "IPv4" set - then if your account has been migrated to "the new IPv6 network" (as in your SIM is set to connect to their new 5G mobile network core (MNC) data centre), then you will typically be limited to 120 Mbps.
There will be no
Carrier Aggregation (ZTE shows this always as "No CA" irrespective of load) so I believe this is probably the limit from a single connection - there may not even be any deliberate limit beyond this technical limitation of a single 5G connection to the mast.
However if you switch to "IPv4v6" then your MTU will drop from 1380 to 1352 (due to the additional size of the IPv6 packet that encapsulates your IPv4 packet) and you will not get a public IPv4 address (instead the network performs
DS-Lite so IPv4 packets get wrapped into IPv6) and more importantly 5G
Carrier Aggregation will be active which will unlock significantly better speeds (depending on congestion).
Obviously having to infer a lot here but I wonder if the IPv4 PDP is just for legacy devices and devices which have compatibility issues with things such as CA. It's a pity that this seems to be the only way to get a public IPv4 address on Three. Could also just be a bottleneck of the 5G IPv6 MNC having to reach back into the 4G MNC SGSN/PGW/GGSN.
A lot of people get confused by the 192.0.0.2 address you get allocated, confusing it with the 192.168.x.x private network (RFC1918) range.. but this is the
correct IP for DS-Lite (RFC6890) and outside the other private network range.
The IPv6 address I believe is public but I'm struggling to get DHCPv6-PD to work (perhaps ThreeUK don't support it). That's a different topic.