welcome back to the Polycom Community.
You would need to figure out of the RPM app can actually connect via the mobile network to your RPAD's external IP.
Most likely your Mobile provider blocks the traffic.
Not sure what connection you are utilizing from your RPD but seems not the same as RPD.
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
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Notice: This community forum is not an official Poly support resource, thus responses from Poly employees, partners, and customers alike are best-effort in attempts to share learned knowledge.
Please also ensure you always check the VoIP , Video Endpoint , Microsoft Voice , PSTN or other FAQ's in the different sections