Thank you @alda for taking the time to do this great step-by-step explanation - I now have the assurance of how to do it correctly! 
However, this is exactly what I have been doing so far so I really don’t have a clue what I’ve been missing here…
Obviously, my EVCS has a different IP address which I can successfully connect to - I also get a green status lamp at the bottom.
Selected data format is also decimal - all settings are alike…
When I set the value to 1 and click on read/write, I also check the bus monitor, where it says “values written correctly”.
Neither setting modbus register 5100 (nor 5101) to 1 seems to “unlock” the phase switching option for me.
I get no feedback from my EVCS that any change in settings has happened at all…neither “EVCS externally blocked by grid operator” nor “internal settings changed, no support”.
When I read the modbus register after supposibly having written value 1, I always still get value 0.
Does anyone have an idea what could be the problem here?
…after a few more tries I just realized that no matter the modbus address I try to read, I always receive value “0” - so there seems to be some sort of problem regarding the access rights I believe, as most addresses sould have default numbers as listed in the VictronEnergy modbus register list.
…when I woke up the next morning it hit me: I had activated the IP address whitelabeling at the very beginning, as it is recommended by VictronEnergy. Well, it makes sense now that I was able to connect but not have rights to neither read nor write the modbus addresses. Never mind - rookie mistake! 