Night time dryness, is a physiological development. The organs/brain/bladder/myelin sheath development developing more. It is different, than day time pottying.
Night time dryness, can take up until even 7 years old, to develop.
And this is normal.
Your child is very young.
My daughter, even at 7 years old, had night time accidents. Normal.
I simply have like 4 waterproof bed pads, from Amazon, that I use under my kids.
My son is 6 and is still wet at night, and is not dry yet. He is also a heavy sleeper etc. And he is in night time diapers. So are many other kids that I know of, that are his age. Our Pediatrician, is not concerned. And, he and I explained to my son, that his body is still developing. My son is other wise an "expert" at pottying. During the day, of course.
Day time and nighttime, pottying are 2 different things, with 2 different timelines.