Keep your existing Gbit routers linked together in your local network and then have a single
Ethernet connection to the Plusnet supplied Router.
That will leave all your local traffic on Gbit links and just the data to/from the modem using what is likely to be a 100Mbit link. There will be a little management traffic too but minimal.
Let the Modem act as the interface, the Plusnet router as a "management tool" for DHCP server &c and your RTN65Us as the networking tool and WAPs.
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