Using a Netgear R6400/R7000 as a client bridge with ATT Fiber gateway

So, we switched from Spectrum cable 200 Mbps to AT&T fiber 1000 Mbps internet (currently have both active, so I can try experimenting with any suggestions no problem).

The modems - modem-only in the Spectrum case and Arris BGW210-700 modem+router gateway in the AT&T case - are in a different room from my desktop, and I want to connect the desktop to the internet wirelessly using a spare router as a wireless adapter. I'm not too concerned with some extra ms of latency vs wired.

My current setup with Spectrum is easy:

• Netgear R7000 connected wired as router to Spectrum modem
• Netgear R6400 connected wirelessly as client bridge to R7000
• desktop connected wired to Netgear R6400 (with internet access at the same speeds as direct ethernet connection to R7000)

But when I try that with the AT&T equipment:

• Netgear R6400 connected wirelessly as client bridge to AT&T fiber gateway
• desktop connected wired to Netgear R6400 (with no internet access)

the R6400 doesn't successfully connect to the AT&T gateway - it doesn't show as a client in the AT&T device list, and internet shows 'disconnected' in the Netgear advanced network settings page. I also have to manually assign an IP address on my desktop to even connect to the Netgear router after I switch on client bridge mode. I've tried resetting factory defaults on the R6400 and starting from scratch, but with the same unsuccessful result.

Are there extra settings on the Arris BGW210-700 AT&T gateway that I need to enable to use the Netgear R6400 router in client bridge mode?

Last resort solution is using IP passthrough with:

• Netgear R7000 connected wired with IP passthrough to AT&T gateway
• Netgear R6400 connected wirelessly as client bridge to R7000
• desktop connected wired to Netgear R6400

But this prevents another upgrade I was thinking of making - using the other of my two Netgears as another client bridge downstairs to connect my TV, Xbox etc. via ethernet to the router that's connected wirelessly to the gateway (hoping this gives me better speeds than connecting each device wirelessly to the gateway).