What modem/gateway is the router connected to? 

Is the phone connected to the wifi for the w121. 

Sadly, they don’t have a gui based access, its app only. 

Hi,

The router is connected to my ISPs router (I’m with BT in the UK).

The phone is connected to the wifi of the W121.

If you’re using the providers gateway/router and then your device, you’re in a double nat. Did you set the w121 in access point mode? Or put the isp’s device in passthrough mode? (a model for the isp router would help)

It’s a BT smarthub 2. There is no passthroug/bridge mode.
I didn’t see the option to set the w121 in AP mode when setting it up. From what I could see, it made me set up a router and then a satellite.

When you were going through the setup using the app, there was an option to “please select your home network configuration”. It was at that step that you would have selected “gateway: cable modem and inbuilt router”

did you select that option? 

Hmm. Can’t recall as it was a number of months back. I’ll reset and go through the setup again to see if I can go into AP mode.

Thanks

Reset both the router and satellite so setup again. Now it won’t go through the setup process. Joins the SSID and then the app says it fails to connect with error 0000-1204. Reinstalled the app. Factory reset and tried both devices. I’ve had this before when setting these up, but it eventually works. Doesn’t even attempt to go through the setup this time. 

May I know the primary modem that is connected with W21?

Please check the internet connection through direct connection via Ethernet cable.

Could you please try swapping the router (satellite as the main router) and try to setup the process again?

http://arris.force.com/consumers/articles/General_FAQs/mAX-W21-Cable-Modem-Internet-Setup

The BT Smarthub 2 is the modem. It’s the modem and router.

Internet connection is fine. I’ve already gone through the steps in that doc and swapped the satellite and router over.

You might check with bt to see if they can upgrade the hub. Thats a pretty old hub 

https://www.bt.com/help/broadband/learn-about-broadband/different-types-of-bt-hub

Don’t really think the hub is going to be the issue here, as the setup fails the moment the phone connects to the onboarding SSID of the w121. Nothing to do with the hub at this point.

This whole setup was a problem a few years ago when I first got these. It’s never been a smooth process. Something that should take 10 minutes ends up taking hours.