NR5103EV2, IP Passthrough, and double public IP addresses

I have a couple of issues. Using Three UK 5G. Works wonderfully.

Firstly, when I enable IP Passthrough only LAN1 works (this is connected directly to the WAN port on my Asus router and all is well)

- LAN2 (which I assume became "WAN" does not work as a WAN port). This coused me a LOT of head scratching till I tried LAN1 out of curiosity.

Consequently, even though it is working , the connections status in the admin page always shows "WAN Status - connection down" - which I can see why its doing it. I don't have anythingb plugged into LAN2 (WAN). Seems to me like the wrong port is being used.

I have IP Passthrough mode enabled, This is "UP" and works. But here is the oddity:

My NR5103EV2 is on Three and is configured for IP passthrough. It works. Right now my public IP is (changed one digit for security)

92.41.37.230

VPN in works, computers see it outside as .230, all my network stuff works
WhatsMyIP gives 92.41.37.230

Mt Asus router shows "connected 92.41.37.230"

My Public IP is 92.41.37.230, there is no doubt.

The strange thing, is to access my router over the LAN to use the admin screen / reboot it etc. I can't put in my browser 92.41.37.230, it doesn't work, I have to put in 92.41.37.229 (one digit below), and on occasions, instead, it's one digit above i.e. it would be 92.41.37.231 (never both at the same time)

What gives?

If I Tracert 92.41.37.230
1 <1 ms <1 ms <1 ms 92.41.36.230.threembb.co.uk [92.41.37.230]

If I TraceRT 92.41.36.229 (the "odd" Ip I manage my router over) :
1 <1 ms <1 ms <1 ms RT-AX86S-2970 [192.168.1.1]
2 2 ms 1 ms 1 ms 92.41.37.229.threembb.co.uk [92.41.36.229]

I'm confused! - how does it have a "real" public IP, and then a second one, that I can uses solely for admin. Are Three issuing 2 public IPs to me? Or more likely is the router generating a pretend public IP for me to manage it?