Bonjour madmoiselle! 
Ok, let us have a look, normaly i would say you are in the wrong forum, cause you are NOT USING any CODE, but you are searching for CODE-SOLUTION, so in this case, we will not reject you.
Ok, what do we have?
- A dynamic page —> means we have also a dynamic —> dataset (O.K.)
- Dynamic-page means one data-value per curent page (O.K.)
- You have also 1x DropDown-List and 2xButton to switch data (previous/next)
Ok, i think there is also a solution without code possible, but not sure.
I can explain you why it is, like it is.
Your PREVIOUS & NEXT-BUTTON will work, because you have connected your dynamic-dataset to them.
Your DropDown will NOT work, because you probably also have connected it with the DYNAMIC-DATASET.
For the DropDown you have to create a second DATASET and connect it directly to the wished DATABASE. Also the LIST-VALUES of this DropDown should be connected with the same DATASET.
DO NOT CONNECT YOUR DROPDOWN TO —> DYNAMIC DATASET!!!
You can see here an EXAMPLE on thsi DYNAMIC-PAGE how the DROP-DOWN, works without any CODE…
https://russian-dima.wixsite.com/meinewebsite/dynamic-action
First ENTER the DYNAMIC-PAGE!