Air carrier Captain reported a discussion with Dispatch regarding an approach procedure made illegal due to 5G and a misunderstanding with flight crew qualifications.

Narrative

The weather in ZZZ1 on Day 7 was showing light snow all day with visibility forecast to go down to 1.5 sm and winds shifting to 360/08 starting at XA00/Day 8. Our arrival for Aircraft X/Day 7 was scheduled for XC50 on Day 8. I called Dispatch at XV37 (3 hours and 40 minutes before departure) to give them a heads up that the forecast was going to be a problem due to the fact that the alert effective on Day 7 for ZZZ1 (indicating that a new 5G cell tower in ZZZ1 was not currently covered by an Alternate Means of Compliance (AMOC) and the RNAV (RNP) Z Runway X was prohibited.) combined with the low visibility would not permit a landing to the north because of the increased visibility requirements of the available approaches (>3sm). The Dispatcher had not looked at the situation since the flight was still almost 4 hours out. We discussed the issue with the wind and visibility; and the possibility of landing to the south with an 8 knot tailwind. I told the Dispatcher that I would not be comfortable landing in ZZZ1 at night with low visibility; snow; a strong tailwind and reduced braking action. The Dispatcher said that he would discuss the issue with the Dispatch manager.I did not hear anything from Dispatch or see a flight plan so I drove into work. I also noticed that my original First Officer (FO) had been removed from the trip and a reserve FO assigned to do the ZZZ-ZZZ1-ZZZ turn. When I got to the airport parking lot; I noticed that there was a new flight plan and reviewed it. The Dispatch release showed that they were planning on landing ZZZ1 Runway X using the RNAV (RNP) Z XX. I called the Dispatcher to inform him that the release was illegal due to the alert prohibiting use of the RNAV (RNP) Z Runway XX. The Dispatcher seemed unaware of the alert and only seemed concerned with whether both the Captain and FO were qualified to do RNP approaches. Once I alerted him again to the alert; he agreed that the Dispatch was not legal. He said that he would speak with the duty manager to discuss options and call me back.The Dispatcher eventually called back to say that the flight would probably be cancelled; but the duty manager wanted to know if I would be agreeable to dispatching with enough fuel to fly to ZZZ1; see if the weather would support shooting the RNAV (GPS) XX Runway X once we arrived; and if not; diverting back to ZZZ1. I told the Dispatcher that I would be okay with that plan if they could build a legal dispatch with as much fuel as possible to give us divert options. The Dispatcher said that he would work on it and get back to me. Eventually the Dispatcher called me to let me know that they could not build a legal Dispatch and that the flight was canceled. After the flight was canceled; I found out that my original FO had been removed from the ZZZ-ZZZ1-ZZZ turn and reassigned because he was not RNAV (RNP) qualified. So it appears that the Dispatcher misinterpreted my original call (3 hours and 40 minutes before departure) as a concern about my FO's qualification to do the RNAV (RNP) Z Runway XX; as opposed to my actual concern that the RNAV (RNP) Z Runway XX was prohibited due to an alert.