I have just received this particular modem and I have put this in bridge mode. Upon inspection of the casing, it is perforated on all side. While the vents on the top are larger than the sides, it would not harm the modem if it was lying on its side (bridge mode only) in a well ventilated location. Ideal is still upright of course.
We've had customers place the modem on its side in the past, with the expected result that the modem overheats which causes problems. Like any other modem, the circuit board is probably populated on one side, so, you have a 50/50 chance of placing that populated side facing up so that it can vent, if not as designed, at least to some point where the modem is stable. If not, facing downwards, you end up with heat building up within the components which causes problems. Despite the speculation, this has been tried in the past, with poor results.
As per @RogersMoin's comment, for anyone running the modem in Gateway mode, the circuit board is populated with three 2.4 Ghz antenna and three 5 Ghz antenna from what I remember. Those antenna installations are designed with the idea that the modem is upright, so that the vertical propagation pattern from those antenna will work as designed. Placing the modem on its side would result in some type of horizontal propagation pattern which will work to some degree, but certainly not as designed.
Fwiw, the Hitron CGN3 modems run fairly cool when sitting upright. Fairly cool is a comparative statement as other modems manufactured by other companies run much warmer, to the point of being hot to the touch.