Have you tried any of the other video outputs in case the one you are using on the Radeon 7 is defective?
If you don't get any video output on any of the GPU video ports, you may want to open a Warranty Ticket and see if the Manufacturer's support site suggests RMA the Radeon 7.
Are you using the same cable for both the Radeon VII and the RX480?
I just put the rx480 in and hooked the power up and no display out and the VII works.
All the outputs are normaal no lag or weird things even my vr headset runs fine of the VII