Dell X1052p - Gateways

I googled etc and already found threads where other users have the same issues. Just no clear solution.

A customer has a Dell X1052p switch. That switch should be reconfigured for another site.

I upgraded the FW, did a reset, logged in via default IP 192.168.2.1

The goal is to have it in a tagged management LAN with ID 99.

So I created the VLAN, configured 2 trunk ports transporting that ID (plus ID 1), and then tried to add a second IPv4 adress on VLAN99 (additional to the default DHCP adress on VLAN 1).

After applying that I lose connection. Every time.

Why?

Maybe the gateways are missing? But where to add, the switch shows these fields greyed out.

A thread talked of "Level 2+ per default" and "no gateway needed". Do I have to add the gateway as a route entry in "Routing"? If yes, what's the right order not to lock myself out?

any help appreciated here, I should get that working tmrw .. thanks!