Can't get AX3000/RAX36S to function as AP

Hi all, hoping that someone is able to help out, as I am becoming more and more frustrated - bearing in mind that anything IT-related is not my speciality! 

What I am trying to do is use my new AX3000/RAX36S (seems to be a very new model) to function as an access point at the other end of the house to the ISP port and the originating router (which is a Nighthawk AX 5/RAX30). I connect the two of them with a standard ethernet cable, going from LAN1 on the RAX30 to the WAN port on the RAX36S. 

When I manually set up the RAX36S as an access point, via the configuration page, it immediately reboots and then becomes impossible to access. No matter what IP address I try, it says "no internet connection, cannot proceed" in Chrome, the same in Safari, words to that effect. I cannot find the RAX36S either through the RAX30's configuration page, despite them being linked by an ethernet cord that runs from the RAX30 to the RAX36S. As in, I figured it would appear on the RAX30's settings page if they are physically paired. I also cannot find the IP address of the RAX36S using IP Scanner on Mac, it just doesn't show up at all, every device that does come up can be accounted for. 

This is despite the fact that the RAX36S can still be connected to, at least on my laptop (Macbook Air M2), on all of our phones it cannot be connected to. Importantly, it doesn't have any internet light displaying, and when it does occasionally flicker, it is orange/red. So really, the point here is just that it puts out a wireless network, but only my computer can join it, and even then, I cannot access the configuration page, let alone the internet. 

So my only option is usually to hard reset the RAX36S to factory settings, which means I can get back onto its configuration page, but again, I want it to function as an AP. If I install the RAX36S using the ISP connection, that normally goes to the RAX30, it installs fine and connects to the internet (as in, if not using it in AP mode but rather in standard router mode). I have tried different LAN ports on the RAX30 and the same on the RAX36S, mostly using LAN1 on the RAX30 and then the ethernet cable into the WAN port on the RAX36S. 

Finally, if this is important, previously I was using the RAX30 which was plugged into an old ASUS router I have lying around. I set up that ASUS router a long time as an AP, and when I use the same ethernet cord between the RAX30 and the ASUS, the ASUS works perfectly as an AP and there is a solid internet connection. If the RAX36S is sitting there, in AP mode and not responding or functioning, and I take out the ethernet cord and plug it into the ASUS, the ASUS immediately connects to the internet and works as an AP. So that makes me think it can't be the cable or the RAX30 that is faulty. Likewise, given the RAX36S connects to the internet fine, when directly connected to the ISP connection, that means its WAN port is working, right? All of this makes me think it's a configuration issue, rather than hardware, given every port on each router is proven to work, both can connect to the internet successfully (individually) and given that the ethernet cable works fine. 

Any help would be a godsend! I'm really at a loss now, I have no idea what to do. Putting the new router into AP mode basically completely disables it. As I said, I've tried at least 15 different IP addresses, none work, and I cannot for the life of me find the RAX36S's IP address... the only thing is that one does come up on my Mac, when I right click on the connection in settings, but pasting that into Chrome or Safari, it just states that it cannot load (rather than with the other 15+ addresses where it states that it cannot connect due to no internet connection). 

I'm just not sure if there are settings that I need to change in either one or both router/s to get this to work? I thought it was as simple as manually configuring the new router, as in the RAX36S, going into advanced settings, changing it to access point mode, telling it to get the IP address dynamically, and then it reboots and works as intended, and can both transmit the internet and be connected to. Sadly doing that hasn't worked, at all.