What put Flynn on the FBI's radar with regard to Kislyak were his text and cell phone conversations with Kislyak -- which were intercepted by U.S. intelligence as is routinely done with foreign officials -- while Flynn was vacationing in the Dominican Republic.
I'm no expert in U.S. intelligence matters, but I'd presume that conducting cell phone and text communications with a foreign official while I'm on vacation in a Third World sh!t-hole is probably the dumbest thing I could possibly do.
His phone wasn’t working in D.R. at the time. Therefore, your premis is in error.
The real problem is Flynn being paid by turkey and not disclosing it —even after trump named him the incoming Natl Security advisor. THAT is what mueller is threatening to charge him with if he doesn’t like the Lying to the FBI charge. And that charge is why Flynn would rather go down on the lying charge than be guilty of being an agent for a foreign govt.