Regular UFO Walls (and floors) have a damage rating of 80. According to UFOPaedia, Heavy Cannon(AP) can only deal up to 42 damage (which is unlikely, since the range is 25-75% normal damage) to terrain, and grenades deal 25 damage (and only to the center space).
The way terrain destruciton works, is that if a weapon deals more damage than the terrain's damage rating, it will become damaged / destroyed. This is a hard threshold, and it must be breached for each attack if you want to inflict terrain damage at all.
So unfortunately, the UFO appears unassailable, at least with your current equipment.
Fortunately, there is no penalty for leaving a UFO recovery mission. If you can't coax the aliens to come out to die, just load up all the corpses, weapons, and artifacts lying around the mission site back into your strikecraft... and leave. You'll miss out on recovered UFO parts, but that's it.