Thanks!

“Untagged” here means default VLAN with id:1 - the one used when nothing is set. By default all ports on the switch are set as untagged for this id.
Maybe i should have marked VLAN1 and VLAN2 as VLAN10 and VLAN20 (with respective id’s) to avoid confusion.

Yes currently its set to dumb switch but i can change/tag them according to my needs. Ive already created VLAN1 on edgerouters side for other devices (unifi AP, pc). So this part should not be more difficult in case of VLAN2. Except trunking. So what im asking here would connecting tagged ports (vlan2) on edgerouter and tagged ports on the switch with second cable work? Or should i still try to push both tagged and untagged traffic to switch through one cable

Even if your login page is not easy to break, it will be indexed by robots or hackes in their list. And they will test on it every vulnerability that will be published for any DSM component. Using VPNs like ZeroTier or Tailscale is definetly MUCH more secure than all of those tweaks and easier to setup too.

But offcourse its YOUR data so … good luck :)