@AAugustin 

Hi,

Does the GUI of the ISP router display some sort of explanation what their "bridge mode" actually does?

The fact that you are having 192.168.100.x addresses, which are commonly used in NAT router scenarios, makes it feel like this isn't really a "bridge mode".

Do you not get any IP addresses assigned at all when selecting "Dynamic IP" on the AX1800 or "Obtain IP address automatically" on your laptop?

By the way, you mentioned you "connected one of its LAN ports to the the WAN port on the AX1800". Do you mean you are connected to LAN port #1 of the ISP router or one random port? Be aware that on modem routers with multiple LAN ports often only the port labeled LAN1 will function properly in bridge mode.

  @woozle 

Thanks for your reply! The ISP GUI is not very helpful, there's a warning saying that once bridge mode is turned on, WiFi will be disabled and will need to physically connect to the router in order to access the GUI again, and it will run on 192.168.100.1 (otherwise, in router mode it's on 192.168.0.1). For the rest, the only things available once it's in bridge mode is logs, changing admin login password for the GUI interface, apply updates & information on the uplink and downlink frequencies.

I don't get any addresses automatically when the ISP router is in bridge mode, making me thinking DHCP is disabled. Neither AX1800 nor my Linux machine were able to obtain a local address, had to set a static IP for both when connecting to the bridge directly.

Sorry about not being clear, I've connected LAN #1 from bridge to WAN on AX1800. I've used the same port when testing Laptop <---> bridge connection.

The part I cannot think of an explanation for is why it works if I give a static address to the laptop, but not when I give a static address to the AX1800.

Also, if I leave ISP's router in router mode and connect AX1800 in router mode and leave it to dynamic, threfore using the DHCP server on the ISP router, it works OK (but of course I want to avoid double NATs!).

  @AAugustin 

I think the purpose of the ISP router's IP address of 192.168.100.1 when in bridge mode is to provide a way to access the configuration GUI of the ISP router. That you somehow get an Internet connection through this when using your laptop is just a side effect. There is almost certainly a NAT involved when connecting via this 192.168.100.1 address, so you are not gaining anything. 

Normally I would expect a cable broadband router in bridge mode to provide IP addresses via DHCP to the device that is connected to its LAN1 port and things should start working automatically.

I suggest you try one more time by following these steps:
1) Disconnect the Ethernet cable between the ISP router and the Archer AX1800.
2) Switch the ISP router to bridge mode.
3) Configure your AX1800's "Internet Connection Type" as "Dynamic IP" and make no additional settings that could affect how the AX1800 connects to the Internet. If you have changed settings previously and lost track of what settings were modified, then maybe it's best to factory-reset the AX1800 router to have a clean starting point.
4) Power off both, the ISP router and the Archer AX1800.
5) Connect the Ethernet cable coming from the ISP router's LAN1 port to the AX1800's WAN port.
6) Power on both routers and wait a few minutes.
7) Check on the AX1800's "Status" page whether it still has no IP address, Default gateway and DNS assigned to it.

By the way. When the ISP router is in bridge mode only one single device (in this case the AX1800) should be connected via LAN. Do not connect additional devices to any of its LAN ports, as this could screw things up.

If it really doesn't work, then you will need to contact your ISP and ask them to provide instructions on how to use their router in bridge mode correctly.

  @woozle 

I'd assume bridge mode would disable DHCP, since the whole idea is for the bridge router to act as a pass-through device to allow two networks to communicate with each other without performing functions of a router, e.g. DHCP.

I've went through the process again and unfortunately it's still not working: no IP address is being assigned when in Dynamic IP (I did a factory reset to be on the safe side). Also, nothing else was connected to the bridged router but the AX1800 (LAN1 on bridge to WAN on AX1800).

I've borrowed a colleague's old router (Huawei AX3), reset that to factory settings as well as ISP's router; set the ISP into bridge mode and the AX3 in router mode and let it automatically set itself up: same behaviour no IP address. After which, I've reset the AX3 again and this time set it with a manual static address, used the exact same settings I used previously on the AX1800 and it worked! I have internet access!

I was a bit perplexed, so I reset the AX1800 to factory settings again and set it up with the same static IP I had used on the Huawei AX3 (MAC address included) and still no luck. After, I disconnected the AX1800 and tried again just with my laptop and same static IP settings (MAC address included) and it did have internet access. Finally, disconnected my laptop and reconnected back the Huawei AX3 which still had the same static IP settings and it worked - it had internet access.

I guess it must either be some incompatibilities between my ISP and the AX1800, or an implementation detail which the AX1800 is not catering for when used in router mode with a bridge. I will set the ISP's router back into router mode and just use the AX1800 as AP, the WiFi signal is much better with it.

Thanks for your help!