The gauge can be inaccurate, depending on how much meat and where the meat is placed in relation to the sensor. Many of us use a separate probe to monitor the temperature, such as the Maverick ET-73. I place my probe under the lowest rack that I have loaded.
I might have missed it, but what vent opening are you using? Chicken with skin has a lot of moisture, so the vent should be 3/4 opened, at least at the beginning until a lot of the moisture is expelled. If you keep the setting too small, that will keep the temperature down.
You can try using a marinade after brining. Some will inject flavor instead of using a marinade.