Turris Omnia - VLAN on TOS 4.x/5.x

That is your default or you mean that being the default by the OS? For the latter all DSA ports are in untag state, check with bridge v, e.g. prints

port    vlan ids
lan0     1 PVID Egress Untagged

  • untag state (one VID per each port only)
    bridge v a dev lan1 vid 8 pvid untagged
  • tag state
    bridge v a dev lan1 vid 8 tagged, or
    bridge v a dev lan1 vid 8 pvid tagged

similar operation for a bridge port exchange the device name, e.g. dev lan1 with dev br-LAN2_IOT.