I am having a rough go of getting VLANs to work on a SG200 (access switch) that has a LAG connected to a SG300 (core switch). The SG300 is in L2 mode, they are using a ROAS configuration. The SG200 will have every port in one (new) vlan.

I have:
-created the VLAN on both switches, ensured it is enabled
-in the LAG configuration on both switch, I have left VLAN 1 as the "untagged" vlan and added VLAN 12 as tagged
-on a port on the SG200 (access) I have added vlan 12 as tagged and left vlan 1 as untagged, tested - no change, the device connected to this port was still in vlan 1
-changed the same port on the sg200 to having vlan12 as untagged, removing vlan 1 completely from it - the device is unable to get an address via DHCP nor is it able to access the network with a static IP

What am I missing?

edit: Something I realized while starring at this, I forgot to enable the lan to lan rule in the firewall that is acting as ROAS...so that might be my issue. Barring that, does my logic above appear to be on the right path?