We used to pop ours on a play mat with arches and dangly toys, on her back, and she did a lot of reaching and wriggling with that. We never did any scheduled tummy time - but she spent a lot of time lying on her front on top of me, and I think that helped her use her neck muscles, as she would look up at me quite a lot. Holding her like this just felt natural and right for us I guess.
I tricked mine into rolling over for the first time (front to back) at about 4 months by dangling a toy and dragging it round so she naturally tipped herself over. It wasn't until closer to 5-6 months that she learned to roll over fluidly both ways with intent. This is a perfectly normal age for these milestones to be reached. She used this method for a good while to get around, before crawling at 8.5 months - walked at 12.
Don't worry too much - a friend's baby the same age didn't roll at all until 11 months or walk until 14, and he now walks and runs better than my (admittedly hypermobile) little lady - he can jump too, which mine can't!