The secure imperial distress channels aren't secure anymore, TNO only happened to have monitored them, not still actively used them themselves. NR elements would have easily had access to old imperial channels nowadays. Despite being neutered itself, I would say in the story angle galactic scope, apart form even whatever Hoppus controls himself, in the AJ story angle the NR is the somewhat prevailing force around. The will be weak, but they will be a joint group, where us all us Remnants of the Remnants are extremely isolated (Bespin, Scarif, Dagobah.. nothing incredibly stand out ish). The NR itself could be considered to be Chandrila, Kashyyyk, Mon Cal, Sullust, Bothan space, at the very least along with whatever-tf Hosnian is/was.
NR showing up at Dath is in story enough just that they are monitoring those channels and are butting in. I bet they could use an ISD!