Hub 5x and connected ASUS Router, not modem mode related (I think)

So I had VM installed yesterday, with a 5x Hub. I was previously with Sky broadband, and switched mainly for the better bandwidth on offer. Before that I was with BT.

Now I've always had a separate ASUS RT-AC86U router for years, because ISP hubs are notoriously rubbish. The ASUS is always in router mode providing Wifi AP on a 10.0.0.x network range, and it's all configured nicely with my static IPs since I have loads of devices in the house. Its been rock solid over the years, even with loads of devices connected.

Then, when switching ISPs, my usual process is to just plug the wan connection of my ASUS to whatever ISP hub I have, with it set to get a DHCP from the hub (usually a 192.168.0.x address), then switch off the hub wifi to remove radio interference, and off we go. This way I get to keep the same custom wifi ssid/IP addresses etc., and there's no drama.

However, trying this with the 5x hub I'm having all kinds of problems - devices aren't reliably connecting to my Wifi router (mainly it seems because they cant find a way to the internet through the router -> hub -> internet) - sometimes they connect, sometimes they dont, or they connect then drop etc. And certain websites eg. reddit just dont DNS resolve reliably.

I've been reading on the VM forums about modem mode (or lack of on the 5x) as the setup of choice, but surely my approach above doesn't need that? With my setup, isn't the ASUS just using NAT to make another extra local network hop to the internet? It's worked fine with other providers.

While I'm reasonably technical with network stuff, I confess I'm probably an amateur when we're taking about NAT and DMZ stuff, but is there some gotcha with the 5x hub that I am naively overlooking thats causing the havoc I'm seeing?

Thanks!