My 2 cents worth.
Yup your right. Mini timer can see #5, Mini timer can see #3, TM751 can see #3, but TM751 can not see #5.
it's always something. I really wish it would pass on the code 1 and/or accept the on/off from the timer. Even better is along with the above the TM751 had a setting for unit code it will respond to.
I am assuming you meant to say Mini timer can't see #5 Mini timer can see #3, TM751 can see #3, but TM751 can not see #5.
Well the mini timer probably can't see #5 because you have the slide switch set to 1-4. Set the slide switch to the 5-8 position and see what happens. While the timer can control 8 units, it can only do 4 at a time. This may be the same reason the TM751 can't see #5 either. Move the slide switch to the 5-8 position and see what happens.
The TM751 is as stated above a receive only module so if you would like to have some selectivity as to putting the unit on a different unit code you should look at the RR501 which is a two way module and it's house code can be 1 or 9 slide switch selectable. Do a search on you favorite search engine for Differences between TM751 and RR501. The site I was on had a table listing the differences.
Well now that I have typed a dimes worth, I will say hope this helps you resolve your problems.
Good Luck,
Bob