Here's a more detailed explanation -
First, download the zip file for the DLC maps. Unzip it.
You should then have a number of folders, called
Freerider_Big_Hollow_Bando
Freerider_Fifty_Fifty
Freerider_MultiGP_Regionals2018
Freerider_MultiGP_UTT1
etc.
Copy those unzipped map folders to the correct folder on your android device.
It's usually:
/storage/emulated/0/Android/data/com.Freeride.FreeriderRecharged/files/
or
Internal Storage/Android/Data/com.Freeride.FreeriderRecharged/files/
If you are uncertain of the folder location, save any level (even an empty one) in the Level Editor of Freerider, and take a note of the full folder path that is displayed after saving is complete.
You may need to use a good file manager (such as Solid Explorer for example) to copy the maps to the correct folder.
(Some phones are deliberately blocking the owner from accessing certain folders, but you may get around that by using a third party file manager).
Hope that helps, let me know if you have more questions or need more detailed instructions for some part!