I have updated my home network:

Before (works fine): Internet provider (fiber) -> Modem (bridge mode) -> Apple time capsule -> Switch -> all my cabled devices

Now (does NOT WORK): Internet provider (fiber) -> Modem (bridge mode) -> tp-link AC5400 -> Switch -> all my cabled devices

Now (DOES work): Internet provider (fiber) -> Modem (bridge mode) -> Apple time capsule -> tp-link AC5400 -> Switch -> all my cabled devices

As you can see, the difference between what works and not is that it seems I have to leave the Apple time capsule inbetween the modum (in bridge mode) and my new tp-link AC5400. I have a dynamic IP from my internet provider, and the tp-link is set up to get dynamic IP address. If it works by leaving the Apple time capsule in-between the modem and the tp-link, why does it not work when plugging the tp-link DIRECTLY into the modem (which is in bridge mode)?

I would LIKE to avoid the Apple device 'in-between' the modem and the tp-link, since I guess a direct link to the tp-link (which is supposed to be a super-fast router) will only slow down the connection speed?Solved: For some reason it was necessary with a reset of the modem, but - and here is the interesting part - it was not solved by me unplugging power to the modem (I tried this several times), it had to be reset remotely by the internet provider. They TOLD me this was a 'harder' reset than I could do by disconnecting the power supply.

So; if you experience something similar, try asking your Internet provider to do a remote hard reset of the modem (I guess this depends on which systems your provider has, but at least it was the solution for me).