No internet connection after new router

I installed a new EE 5G wireless broadband router and there's no problem connecting it to all our apple devices: three iPhones, a ten-year old iMac, a mid-2015 MacBook Pro, a brand new M1 MacBook Pro and and a new M1 MacBook Air. The problem is really weird: with all of these devices except the mid-2015 MBP, there is no problem with connecting to the internet, but with the 2015 MBP, there's no internet signal, even though it is connected to the router just fine.

To be clear: each of these devices is connected to the router and gets a strong signal. But with the 2015 MBP any attempt to browse the net just gets "you are not connected to the internet" or sometimes "the server has stopped responding". There is no email either. The other devices connect to the same sites just fine and get email as normal.

The 2015 MacBook is running the latest version of Monterey (as are the other MacBooks), and the old iMac is running High Sierra. The iPhones are all up to date. Naturally i have tried starting and restarting the MacBook and the router.

Any suggestions welcome.