You may not need to do it that way. Most of those setups should roam on their own. But even if yours doesn’t, you could just toggle WiFi on and off to connect to the strongest signal. (Which would be why I have exactly that pair of shortcuts saved on my home screen.)
The functionality should be the same no matter where you are. Unless using Eero is banned in Denmark. I don’t know about the laws of Denmark. You can check with your ISP if it’s illegal for you to connect your own router to extend your WiFi within the home.
The way I have set it up is connect via Ethernet from ISP router to Eero. Disable Wifi capabilities of ISP. Eero becomes the Wifi provider all over my house.
My ISP had no issues and they were totally fine with it.
Another thought might be to check if your ISP makes nodes that are specifically designed to work with their router. I know Comcast has that in the States, but I still went with a Linksys WiFi6 system recently for the insane speed.
Just throwing ideas at the wall to see if something sounds like a plan.
There is a good reason to disable the ISP Wifi Router. Though its optional. Chances are you will have both the ISP router and the Eero (or any other Mesh router) side by side. They both will be in the range 90%-100% Signal strength. When you drive into your home, your mobile device can pick the strongest and chances are it can pick the default ISP wifi SSID too. You can disable to forget it, but you will have to remember to do this on all your mobile devices.
Also too much of wifi signals beaming is unwanted noise. More security issues and headaches.
Also most ISP routers (at least in USA) try to enable the public wifi by default on all their routers. By disabling the wifi you will entirely keep it only to yourself.
Again as most you said, you can always leave it ON. Its optional