This is a success story on how I repaired 4 IR wireless handsets I gave up on 10 years ago.
They all work now.  I bought all these off Ebay over 10 years ago.  I suspect the reason I got them so cheap is because they were not working right in the first place.  So, these are the worst of the worst.

Repairs were all done on the back of the board.  It looks like they (China) didn't clean off the flux after they mounted the thru-hole parts.  Dendrites are fine crystalline hairs that form in humid conditions using residue underneath layers of old flux.  They lead to shorts or resistances that mess with microprocessors.  In short, it's poor workmanship that leads to infant mortality.
The two pictures below, one is a controller where one pair of batteries goes dead overnight - powered off.  Another is the IR receiver signal line that only works for 5 minutes after being turned on - then it disconnects.
After repeated cleaning cycles of scraping off the old flux, rinsing with solvent alcohol, then using compressed air were these cleaned into reliable operation.  Keep doing those steps until all the white sticky residue is gone.
The backs of these units were held on by 4 screws (longer ones to the battery case side.)

It's also important to observe ESD precautions.  If you are not set up to work on sensitive electronics, do the work on 50%+ humid days, wear cotton, and ground yourself to a metal cabinet or pipe that is also grounded.

The defects found are usually the controller going nuts like a three year old child running the joystick.
A different problem is the unit shutting off when bumped.  While you have these apart, clean off the battery contacts with alcohol.  If that doesn't work (like if a battery leaked out), use a small file to get some shiny metal.  Wipe on some machine oil to keep the filed contact from rusting.  Be sure and also check the removable 'Lid' contacts for any corrosion.

Missed one.  After reassembling and testing, this one worked but had a strange problem.  I had to press hard on the joystick right and it always jumped several locomotives each time.
Opened it back up finding I missed some dendrites shown below.  Cleaned it and it works OK.
On a different controller, I had cleaned these battery contacts.  It worked two days ago, but not today.  I successfully tinned the positive terminal, but the negative spring was too far gone.  Filing took off half the spring.
So, I covered it in solder braid, draped over the spring.  The solder braid is soldered to the PCB.

After going through all my Dynamis stuff, I thought I was seeing things.  I really have TWO dynamis control boxes??  They both work OK.  I also have two of the EZ controllers with the dial that can only talk to 9 DCC locomotives (ID numbered 1-9).

I have no clue where I picked up the second Dynamis system.  I was probably buying more hand controllers for $30 and they threw this in for free.

I tried the 'extra' dynamis command station on my 4x8 sheet of plywood test track and let it run for about 30 minutes.
After that time, both handsets lost signal and the trains stopped.
I swapped out the IRDA head and it worked OK for the next two hours.  This is likely why I got the command station so cheap.

The ROOT CAUSE of these Dynamis failures is not so obvious.  Yes, the Chinese work is sloppy, but also the German Engineering also has a fatal flaw that allows this sloppiness to cripple these good products.

The BLUE Plastic layer that surrounds the solder pads is the WRONG SIZE !!!!  It should allow at least 0.020 inches of space around those solder pads to keep the flux and other soldering waste out from under this 'Solder Mask'.

Instead of protecting the PCB, it funnels solder flux UNDER the solder mask.  This forms a petri-dish of Dendrite Formation.  The hand-soldering of the connector separates and damages the solder mask.

The repair is to carefully remove the solder mask from around these pads with a sharpened small jeweler's screwdriver, then clean and apply a sealing RTV over the whole set of pads to keep out moisture.

It's a tragedy that Dynamis was so discredited for reliability because some engineer goofed on the size of the Solder Mask through-hole openings in the PCB Gerber Files.

Initial cleaning involves reflowing the solder to eliminate all the dendrites already formed.  It also heats the area to remove any moisture.

A good cleaning with Solvent Alcohol and compressed air removes the flux residue.  A good scrubbing with a Dremel with rotary wire brush also cleans up the solder mask.  Additional cycles of Alcohol, Compressed Air, and microscope inspection cleans it off.

Note in the first picture how close the blue solder mask surrounds the solder pads.

I didn't remove the solder mask on this one.  After getting as much debris out of the gap as I could, I cleaned it again then baked the circuit board area with a heat gun to evaporate any oils or water under the solder mask.

Tested for 1.5 hours -- no loss of signal.