Tweaked the router a bit, optimizing it. And succeeded getting the computer having a Direct connection through the router.
Tried a big file transfer and make the router with Zerotier sweat a bit, I saw the router cpu usage go to almost 100%.
So it’s clear to me now the cpu in the router is a bottleneck.
I didn’t know yet the router cpu played such an important role for transfer speeds, but I allready want to upgrade it 
Do you maybe have a suggestion what kind of minimum cpu I would need in a router for Zerotier to work effortlessly? Like minimum Mhz and amount of cores?
Thank you for your answer!