Hello,
I would like to disconnect my server from the rest of the network as shown in the picture.
Is this possible with the “MikroTik hAP ac²”?
Is it possible to make the server reachable from the internet?
Is it possible to configure that I can access the Vlan 10 from the Vlan 20 but not the other way around?
What would happen if I connect the server without Vlan to the router?

In order of asking:
1- yes but obviously depends on how much traffic you plan to route through that device.
2- yes, port forward would be the easiest way.
3- yes, no problem doing so.
4- depends on your config but if done right, anything connected to that specific ethernet port will end up being on VLAN10 and nothing else. No need to define those VLANs on the server. It will not even know (and it shouldn’t).
Thank you for the answer.
There will not be much traffic on it. Just an occasional connection from me to the server, from my husband and my mom. There is only one private Nextcloud running for the 3 of us and one private recipe database. So everything very small 