yealink W52P not working when connected to my xr500

Hi, I'm having trouble with my yealink W52P  DECT phone when connected to my XR500. at the moment, i have it working by connecting it directly to my ISP modem (which is in router mode, I previously had it in modem mode). 
but i just cannot get it to work via my XR500, is there any troubleshooting i can do i investigate this?

Why not just leave it this way you ask? well, hybrid VPN doesn't seem to want to connect with this configuration :(

I've talked to my SIP provider and while connected to the XR500, they get no signalling from the device at all, i figure it's a firewall type issue and have forwarded every port mentioned on the device's browser interface to the phone, but still registration fails. 

I've disabled SIP ALG and checked the MTU.

I'm not sure what else to do!

Thanks in advance!