Won’t start

So I recently bought a 1998 Chevy 2500 with the 350 and it won’t start I bought a new battery and got the alternator tested the thing cranks over but won’t start I think it’s because it’s cold and gets in the negatives at night but it’s annoying sitting there in the morning with jumper cables waiting for 20 mins. I also unhooked the battery last night and hooked it up this morning and still wouldn’t start