15 hours really is what I would expect for about a 12lb brisket. And cooking at 235*, you are going to get a slower cook. Brisket isn’t as bad as pork butt though. But, sometimes you just get a cut that takes a bit longer.
That said, if you are wrapping in foil, you can always move the brisket to the oven and finish it at a higher temp, upwards of 325 even. And, last - use the internal temp as a guide but go for probe tender and seeing if the brisket moves like jello. Those are two other guides to follow. But, with it only being at 183*, I wouldn’t even had started to check for probe tenderness.
But, hopefully you get the thermo calibrated to help. Maybe it was closer to 190 if it was off a bit??
Hope that helps! Also glad you were able to enjoy that brisket in the end!