Hi! I wanna find some programm for Ipone 6s for monitoring NXDN. can you advise me something?
The iPhone does not have a radio receiver that will allow you to listen to public safety/land mobile radio frequencies.
There are ways you can set up a gateway on the radio system and connect to specific applications on your phone to allow communications over the radio system, however that requires access to the radio system. Unless you are the system administrator and/or can legally add radios to the system, that's not going to do you any good.
As ka3aaa said, you'll need a scanner with NXDN support to do this. Or, find someone who is streaming the system you want to listen to.