hey want to use 12V for more Range… and i use a “Duracell MN21 V12”(https://www.amazon.de/dp/B002NRQO2Q?psc=1) its the same like in the standard 433Mhz Remote Controllers.I tried also a battery of my 433mhz Remote controller without success.
Its a Alkaline battery, which still can give a bit more current then 12v.
I am by no means an expert on using battery’s so maybe someone else knows the answer.
Is it possible to try a lower voltage battery?
This way you are sure the batteryholder is not the problem.
As an alternative for range extending you might want to look into using “ESPimaticRF”.
For that you would need minimal 2 NodeMCU’s and 1 send/receive kit.
ESPimaticRF sends RF through your wifi so you can place it anywhere you want.
The great thing is that you can add another NodeMCU with an RF kit if the range isnt enough!
I have pimatic upstairs and 1 rf device in the livingroom and 1 in the backyard, works perfect.