Hello,
I’m simply trying to apply some VLAN tagging on a port on a 5009 router.
Computer A which is not VLAN capable is connected to port 5 where I want to have VLAN 105 applied on it which then will have some DHCP lease provided by the router on VLAN 105. This port does not need to belong to a bridge.
This simple thing seems to be very difficult to achieve 
Thank you very much for your help.
Please describe the topology in more detail. Where else is the VLAN 105 present?
Only one device has this VLAN for the moment with his own IP range.
Computer A connected on port 5 on Mikrotik, that’s it where I want to be sure the VLAN105 is set.
And adding another port to same VLAN wil never happen?
The thing is that having VLAN set for single port (which doesn’t carry any other VLANs) adds unnecessary processing overhead. It’s better to remove port from any bridge (if I understood you correctly that’s already done) and set appropriate IP address directly on that interface. Also add any necessary service (DHCP server, etc.) to that interface.
Concur, draw a network diagram of all the ports and subnets etc, without context and a plan, advice can be confusing or sub optimal.