Programming Issue - Lionel 6-38045, #5418 LionMaster Hudson

Make sure the wireless tether is not bent and it is ‘seeing’ the infrared light across the tether.
Also, open the tender and make sure the R2LC board is plugged in firmly. As for the sound and amp board there, check to see they are firmly seated.
Also, check the loco handrails as that is the antenna for command and they are NOT touching the real shell.
Yes, #74 is the reset number for wireless tether on steam.

Thanks Ted -- I'll re-check the handrails although I assumed they were not touching as direction and speed functions worked fine.                                                     Is there any specific way to verify correct alignment of the wireless tether?

I would definitely check the 3 plug in boards in the tender. I think there are the sound, amp and r2lc boards there. I would remove and re-seat them in the 24 pin board firmly.
It really sounds like a loose connection someplace. If you open up the engine and remove the shell, check the antenna wire from the handrails to where it is plugged in. Also, I would check all the other plugs into the motherboard there that they are firmly secured.
Something is loose?

Do you have another tmcc steamer you could swap engines or tenders with?

This will help narrow down where the problem lies.

Take your phone camera and look at the locomotive drawbar UV transmitter in the viewfinder while the engine is powered on.  If you see a purple glow, the issue is most likely the tender, first place I'd look is the wiring to the UV drawbar receiver, it frequently gets broken at solder joints and would leave the tender in conventional mode, which sounds like what you're seeing.

If you don't see a purple glow from the engine UV transmitter, then you have an issue with the locomotive.  Same though applies for the wiring under the heatshrink.

If I suspect the wiring under the heatshrink, I normally just one at a time tug on them gently, if one comes free, you have usually found your problem.

Try 740 not just 74.