AMD requires the use of the AMDGPU driver for RX 400 series support. All older GPUs are best supported by the old FGLRX driver. Because of this, you cannot mix and match the RX 400 series with any other series in the same rig.
To switch to AMDGPU, follow the below instructions:
Open your config for editing:
If in single-rig mode, add globaldriver amdgpu to your config.
If you are using remote.conf, set driver [worker] amdgpu to your config, and then run putconf ([worker] is your 6-digit rig name).
Wait for the reboot (if a reboot does not catch within one minute, run ethos-switchdriver amdgpu)
After your rig reboots, you will be able to use the RX 400 series.