Setting up router in DMZ of 5G hub

Evening all. I'm a Three UK 5G home broadband customer but the WiFi from the box is pretty shoddy so I bought myself a proper router (ASUS RT-AC86U) to handle all networking duties. Everything is running at this point but occasionally it drops out and I'm not sure if that's my fault or Three's so I was hoping someone would be able to tell me if I've set it up properly? I know a decent amount about home networking but this is definitely as much as I'm comfortable handling and the other forum posts I've seen tend to confuse me further. My questions are at the bottom of the post.

Currently, the 5G modem (Huawei H122-373) has DHCP turned off and the ASUS router is sitting in the DMZ with an assigned IP - the modem does not have a dedicated bridge mode ad:. The 5G modem has WiFi etc disabled so at this point I assume it just takes the internet and funnels it off into the DMZ.

The ASUS router has the WAN address set to the same IP as the DMZ on the router and is set up to be the main router rather than just an AP. I have no idea what the DNS is doing in the ASUS router and the whole gateway situation is something else that completely goes over my head so I have no idea if these settings are correct. I can connect to both the settings pages for the modem/router but only if I use the IP addresses for them. If I try to use "router.asus.com" this doesn't always seem to work, and the modem wasn't letting me access its settings pages earlier (192.168.8.1). Otherwise, it usually works fine. Until it doesn't!

Questions:
1. Do I leave the firewalls enabled on the modem or should I switch these on in the ASUS settings instead? Does this matter?
2. Could it be a dodgy DNS setting somewhere that makes my internet hang for 5-10 mins or is this more likely to be an ISP thing? I know that the signal to the modem is good here which is what made me wonder.
2a. I have noticed that pinging 8.8.8.8 from the ASUS settings continues to work even while the internet on connected devices etc hangs. Does this mean the problem is somewhere on the ASUS side of things?
3. I can connect to the settings pages for modem/router by IP address or "web address" but these don't always work. What is that about?

I can't think of anything else to ask - as a rule it works okay but the connection hangs every other day or so for reasons that I don't fully understand. Any help would be really appreciated!

Cheers,
Dan