A really strange one, this; recently had to replace a defunct Tenda setup with the Deco PX50 setup - three nodes which are used solely as wifi access points connected to an existing ISP's router with its WiFi turned off.

The router is set to 'bridge' mode so only acts as a modem, rather than a router - the Deco is connected via PPPoE and acts as main router in WiFi Router mode. 

It all works fine, apart from one device. It's a 2012 Macbook Pro, which for reasons which completely escape me, won't connect to any Google service, and only intermittenly to other sites. The MBP is my old one, which my son uses for his homework, and so at present it can't connect to Google Classrooms, which is what his school uses to set homework (we're in the UK if that makes any difference).

My wife has a slightly newer Macbook Air on which it works fine; I have a new M1 Macbook Pro and no problems at all. Every other device we have connects fine, including my work Windows laptop, our phones, tablets etc.

I presumed it was a DNS issue on the 2012 MBP so set it to use either 8.8.8.8, 8.8.4.4 or 1.1.1.1 but still no dice. Pings show it can connect to google.com, but wouldn't resolve at classroom.google.com until I installed the new ISP router/modem. After that, it pings the relevant IP addresses fine but won't load the sites.

We can't log in to any Google account - it throws up a '400 Error' each time or just hangs. Traceroute shows it resolves to Google's main server and the Classroom one, but won't load the sites.

It connects intermittently to other websites we've tried (e.g. BBC). We've tried Safari, Chrome and Firefox, all to no avail. All cookies etc flushed.

From the Deco end, I've tried turning Fast Roaming on and off, given the MBP a dedicated connection to the main node (so it doesn't try any of the others) tried 2.4Ghz and 5Ghz (it seems happier on 5Ghz so left it on that for now), turned mesh on and off for it (no difference). We've deleted the connection a number of times and reconnected - no difference. I've turned SIP ALG off and on (no change).

I thought it was a 'double NAT' issue as the original ISP router we had couldn't do 'bridge' mode, but we'd been sent a newer one which I hadn't bothered to install with the Tenda mesh as at the time it was all working, so have installed the newer one which I presume has done away with any double NAT issues (since only the Deco is functioning as a router now). 

It works on the Guest Network with seemingly no issues. 

Can anyone offer any other suggestions to try?