What kind of switch is that?
Generally, I would start by changing the management VLAN on the switch. Lets say 50. Assume you have this in a lab. Set one of the ports Untagged Egress (Native Vlan) to 50 then set the switch’s management vlan to 50. Now you can manage the switch on VLAN 50. Connect another port to something like an A5 and set its management VLAN to 50 and allow 50 on whatever port you plug it into. Set a C5s management vlan to 50 and connect it to the A5. Now you should be able to manage the switch, A5, and C5 on vlan 50. Now allow some sort of payload vlan on the port the A5 is connected to. Lets say 100. Set the C5’s ethernet port to VLAN 100. Connect some sort of router to the switch and create a payload vlan 100 on one of its interfaces. Connect it to a switch port and allow 100. Now you should have a management network on VLAN 50 connected to your computer, and a Payload VLAN connected to the router. You could also add a vlan 50 if on the router, allow it on the connected switch port, and manage it that way. If your doing it that way, I’d also add a firewall rule on the router to not let the payload vlan connect to the management vlan. I haven’t gotten 2.4.0 in the lab yet but when I do, I’ll do a write up on vlans and particularly QinQ VLANS if I can get them to work right with Mimosa.