Please try WireGuard.
Connect the WAN port of Slate to the LAN port of TP-LINK Omada, then set port forward on Omada. WireGaurd use port 51820 by default. Set Slate as WireGuard Server.
For your cell phone and PC, just use the official WireGuard app/client.

I think this is the best solution and don’t need to buy a new device.

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