Thank you 🙂 I've ordered a plastic exo terra breeding box. I've got a 11inch 14w mat. So would probably have to put it under the plastic tank to one side. Don't see how I could properly attach it to the side of the smaller tank safely. At least off to one side he has a cool side to the substrate to burrow and cool down.
Seems to be very split feelings on whether you can or can't put them underneath where ever I read.

It's ok under just one end. On the side, it won't provide heat properly as the heat will rise & not heat the tank. On the bottom the frog can go on & off it as he thermoregulates.

So today after still not seeing him eat and after taking all your advice ive plopped mr nib nibs into a smaller house.
Will be keeping an eye on him 🙂 p.s thats only a 5.0 repti sun for uvb as he's now tucked away in a bit of a dark spot. Gives off near enough no heat.
Thanks again! Will keep you updated.

I dont believe that Ceratophrys need uv. More importantly is a heat source.
Mats do not warm the air, so unless in a warm room., it won't help much. They should also never be used with a deep substrate as this leads to thermal blocking which can in turn cause the mat to overheat.
I suspect that the overall ambient temperature is just too low. Why not replace that uv bulb with a spot lamp, low wattage, to actually warm the air.

My pacman is on a mat under a quarter of his tank. The mat is under 1" of substrate, the rest of the floor is under 2" of substrate. He goes on the mat when he wants to. & he feeds all the time.