Modems work (and work produced heat), depending on connection efforts. I've temp-gunned some routers that peak 140º in the upper cabinets.
For this reason, most of us will keep those cabinets open while in the sun or even storage. Despite the Newell overhead being fan-cooled, it still gets pretty warm up there. CradlePoint's have heat-sinks all across their bodies, whereas your PepWave probably doesn't. But...if you temp'd it, I'll be it's not as hot as you may think.