Replacing Grohe wireless shower control unit

[Cp1307]

Hello
I need to replace my Grohe Geotherm shower cubicle mounted control unit.
Its around 17 years old but replacements seem to be available
The main hot / cold water control unit is in the loft, I think its working ok
How do I get the replacement cubicle control to connect to the loft unit wirelessly ?

Also the small remote has stopped working, not too worried about that

TIA

 

[Basher]

If you search online for the manual of your particular model, you should find the info you need.
I don't know if I've got the correct Geotherm model in the example below:

I assume the 'base unit' is the bit in your loft!