Adding full stops (dots) into existing cell information ?

Hi all.

I've been using Excel on a basic level for years, and only now am starting to get more involved and interested in the finer details and options available

Todays issue is that I have a sheet with around 4000 rows of information, and the first cell has a 12 digit alphanumeric entry that I want to seperate with dots.

For example it might now say 12138944G00, and I want it to look like 121.3894.4G.00

The dots will always be in the same place, and there is no formatting or auto sum etc in the original cell.

Can anybody offer some advice on the above please?

Thanks.