Depending on where the problem is, I have put a hose into the line, pushed it down to the clog and run the water. Even cold water is much warmer than ice and the water shooting out the hose onto the clog also wears it down. Once it is free, keep running water with the hose to get rid of as much as possible. Make sure you have a back flow prevent on the faucet and you don't use the hose for much else except sewers after that. You also have to have a place for that water to drain as it backs up, until the drain is open.

I have also forced rock salt and brine into the line, let it sit a bit, then go for the hose if that doesn't work. Some line jetting companies or plumbing snakes can get at it, but with all the same issues above.

This is at a rental that belongs to a friend, not one of mine. I haven't actually seen it, but my understanding is it is under a concrete garage floor with access through a drain in the floor. It is now backed up through the drain.

We went to Home Depot and bought a ZEP product I found online that is supposed to thaw frozen sewer lines. I also loaned a heat gun.