Cat C7 starts, idles, but won't rev up in gear.

I had a MH in my CG with the similar problem on a Cat engine. I forget now which sensor's plug (near the fan) was dirty and cleaning it solved his problem. May take hooking it up to a computer to see which sensor is not working correctly or try cleaning all of the two wire connectors you can get to.
A Cat/Allison will do the same thing until it warms up but doubt it's cold enough for it to do that. Worth giving it more time to warm up and see if it will accelerate after a couple minutes.