Can anyone tell me if you need two dongles, one for the P4 RTK remote and for the base station?

Where do you get them here in the US?

For some reason the dongle is not included in the US, but it is included in the EU. I'd recommend you contact your supplier to see if they can reach out to DJI directly. I have not yet been able to find any information regarding compatible 4G dongles.

You might get a better response if you post in the ground station section:-

https://forum.dji.com/forum.php? ... peid&typeid=311