When you export a map to Word the Microsoft Word Export Settings appears. This has a Word Template tab; click on this and then on the arrow in the Word template field to access user templates.
If you want more control over the process have a look at the WordX add-in from Olympic Ltd. This not only gives you access to user-defined templates but also allows you to apply specific styles to individual topics or groups of topics irrespective of their position in the map hierarchy.
If you initiate an export to Word from MM you should get a dialogue box for Microsoft Word Export Settings. If you click on the Word Template tab the first field offers you a choice between User Templates and Built-in Templates, as shown below:
If you select User Template you will be taken to the default Word folder for such templates, which is usually something like C:\Users\Username\AppData\Roaming\Microsoft\Templates. Make a note of the folder name and path or copy it, then close the dialogue box.
Then return to Word and save your template in the same folder. You can now go to MindManager and start the export process; the Word template you created should now be available.
That's very interesting - and obviously very annoying for you. Now I can see why you were having such difficulties!
I think you should put in a support request to Mindjet/Corel as this is a definite bug. You could also look to see if the latest MM 2018 version available for download is more recent than yours and if so if the problem has been resolved (it's not mentioned in the update notes, but nonetheless it may have been fixed).
The latest MM 2018 version on the Mindjet website Is listed as 18.0.284 which on paper makes it older than yours, but I suspect this could be a typo and it could be 18.2.284 - though don't take my word for it!
As a temporary fix you could find and save your current Word template, which should be something like Normal.dotx, under another name and then save your customised template as Normal.dotx. MM should then find it. When you have finished you could delete the temporary template file or give it another name, then save the "real" normal template under its original name.
Good luck with it!