Hi Giorgos,
Before moving on, please check if you have imported the list template in the specific site collection successfully.
The detailed steps are shown as below:
1.Login to Office 365 portal
2.Click Manage under SharePoint in the middle pane.
3.Click the site collection where you imported the list template.
4.Click Site Actions in the upper left corner.
5.Select Site Settings
6.In the Galleries section, click List template.
7.Check whether your list template is there, if not, please upload the template.
8.Create new items to see if the template appears.
Thanks,
Linda Wang