How can I get all transactions for a specific address using Nbitcoin?
you can't get all the transactions done on an address using just Nbitcoin. you need a block explorer for that. you can check out qbi t, blockcypher etc.
However you can get that using Nbitcoin and Nbxplorer (a simple blockchain tracker that monitors your xpub and addresses) with easy to use api's like getTransactions, getBalance etc.