Zyxel VMG3925-B10B Setup for Sky Fibre

Hi everyone, would really appreciate some help in setting up my newly acquired modem please.

I have Sky Fibre broadband (up to 24MB) and i'm trying to completely do away with the Sky Q router, as it's rubbish. I have purchased a Zyxel VMG3925-B10B after reading some rave reviews and my intent is to use it in Bridge mode with an Asus RT-AC66U_B1 as the WAP. I've turned it into bridge mode for both the ADSL and VDSL connections in the GUI, i've also turned off the IP4 and IP6 firewalls. Before and after doing these settings, the internet light on the modem is a solid read, not sure if that's to be expected or not? I've tried connecting the Asus router to it WAN port to WAN port, plus LAN on the Zyxel to WAN on the Asus, not sure which way round it should be but neither works. I've flashed my Asus router with the Merlin firmware and specified a generic sky email in the Option 61 field, so I believe that's setup as it should be.

I'm quite technical but not really when it comes to networking, could someone please help as i'm desperate to get this new combination up and running as homeworking has become very difficult for my wife and I while using the Sky Q router. The main issue we had was that internet was fine while only one person was using it, as soon as another request comes in for bandwidth it seems to cripple the other connection.

Any help would be really appreciated!

Thanks,