Keypad won't learn PIN

The battery on the 976LM wireless keypad for my LiftMaster 2265 died so I replaced it, but the opener wouldn't respond to the keypad so I tried to program a PIN again. Repeatedly went through the learn process to no avail - the opener will not connect to the keypad to sync a PIN.

I tried both learn methods (pressing the learn button, then entering a PIN and holding enter, as well as the procedure by using the wall buttons) and neither worked. Tried many times and figured that the keypad unit itself may have gone bad so I ordered a replacement unit, the 877LM, which LiftMaster indicates is the (new) correct keypad for my opener. 

I pulled the battery from the non-working 976M and put the unit in the house, far from the opener, then attempted to program a PIN with the new 877LM keypad. Nope - same behavior - absolutely no response from the opener. The steps I'm taking are: press orange learn button on motor, enter a four-digit PIN and press/hold enter on the keypad (the buttons lights so I know the battery's good) but the motor never responds with blinking lights or clicks, and eventually the learn light goes dark.

I even reset the opener completely and used learn to program the normal button remotes - worked instantly and perfectly for both. So I know the opener is transmitting new codes properly to the remotes, but for some reason it won't communicate with the wireless keypads any longer.

Any ideas? Thanks much.

The 877LM Keyless Entry is not the correct one. That one is for MyQ Security+2.0 operators only. The correct replacement keyless entry is the 877MAX Keyless Entry. It is backwards compatible for Liftmaster/Chamberlain/Sears/Raynor operators manufactured since 1993.