I assumed that - and it used to be the case, but not now.
I have tried ping and it cannot ping the PlusNet router.
The PlusNet router has a 4 port switch on the back - the MX64 is plugged into one of those ports and if I plug my laptop directly to another of the ports I get access.
This suggests that there is not a firewall issue in the PlusNet router as it would block access via all ports.
I have ensured there is minimal firewall blocking on the PlusNet router on the basis firewall activities are done by the MX64 and having the PlusNet router interfere could cause hard-to-trace issues.