I didn't have any 4g when I was there earlier this year, you'll have to go to Twentynine Palms, its not too far of a drive from Belle.

Inside the park I got decent ATT LTE/2 bars service at the Ryan Mountain trail parking lot and at the turn to Live Oak Picnic area, possibly at the picnic site but I didn't go there to check. Sometimes I got a flicker of signal around Jumbo Rocks but the other two were consistent. Absolutely no signal at any of the campgrounds. I had a need to check my mail or probably wouldn't have even looked.

Getting better all the time. There is a pullout a two minute drive down toward the park entrance (after the intersection) where there is always a signal.

We had significant electrical issues with our RV and had to go to the north entrance on the way to Twenty Nine Palms several times to get service but had it right north of the entrance.