Hi,
Generally, the default Outlook Templates (.oft) is located in drive:\Users\<username>\AppData\Roaming\Microsoft\Templates.
Firstly, please make sure the Templates have been located in the C:\Users\%%\AppData\Local\Microsoft\DRM\Templates path of your local machine. If it is there, please copy and paste the new Templates to the default Outlook Templates location.
Then we can right-click the Folder to select Properties in Outlook, and click dropdown next to "When posting to this folder, use:" and select the custom template form in the list. For more information about Outlook Templates, please refer to:
https://support.office.com/en-au/article/Save-time-with-templates-in-Outlook-8990002c-b3d7-4fc0-a4f4-4f2f94643aa7
Regards,