Services like unihomes split your annual bill into equal monthly chunks and often as a fixed price for unlimited/average usage meaning your roommates not wanting to pay bills over the summer would lead to higher bills later on most likely since they operate on 12 month contracts, either way they'd still have to pay the fixed daily rate for gas, electricity & water etc whether the home is inhabited or not its only the usage that would increase, unfortunately its one of them things that just comes with renting a house whether you live in it or not. If they don't want to pay for the usage charges over the summer months then you could offer to pay a bit more when your bill comes through from the supplier in September/October but they should still pay their share of the daily standing charge.
You sign up for a house and have the costs that come with it I really don't get what they expect I wish I didn't have to pay bills for months im not living in a house but its a fact of life, I had housemates who paid for bills and utilities during the 6 months of covid lockdowns when they were a 1000 miles away from the house with their families and only two of the 5 were still at the house.