the best way to heat dubia roaches is with a heat pad with tiles under the tub they are in best temp for breeding them is around 80 degrees F, also a quick good tip is give them a high protein diet like the bug burger from repashy and the repashy hydroload, if you dont give them a high protein diet they will start eating each other. make sure you have a bunch of egg cartons in the tub so they can hide (24x24 egg flats) so they can adjust their temps.
this is they way i breed them and i have a crap load in like less then a month and i started with about 35 roaches and that gave me about 350 babies plus.
remember temp and a high protein diet is key to the breeding to them!!!!