@sercasti 

Its really your choice, but in my opinion.. switch it to Access Point mode

Yes you are going to disable the router features, but so what.. the AX21 is a fantastic WiFi device and will be better than anything you can get.   The fact this device can offer a router feature is the bottom of the list of selling points for the hardware, its all about the WiFi..

Just my 2 cents  :)

  @sercasti 

Hi,

Do you have access to the configuration GUI of the ISP's modem? (I assume this is a combined modem-router device, is it?)

If yes, then another option would be to run the AX21 in router mode (to be able to enjoy all of its router features) and configure the ISP's modem to forward data packets of incoming connections to the IP address of the AX21's WAN/Internet port. Either via Port Forwarding or by using the so-called DMZ feature.

Yes, this will create the infamous "double-NAT" condition, but this is not necessarily as bad a some people claim it to be, especially if you know how to deal with it. I have been running such a setup myself for a few years now, because my ISP's cable modem also can't be switched to bridge mode, and during these years I haven't had any "Dam*, I can't do this because of my double-NAT!" moment yet.