Hi there!
Thanks for your query.
If you want, you can use one of our onepage templates to get anchor menus working within offcanvas.
As you can understand the Helix Ultimate Template is not made with onepage facility, so it happens. And customization support is prohibited here.
However, I can give you suggestions/hints only. You can have a look into the following video to see if it helps:
https://youtu.be/CBZsdvQShhk?t=509
And the following conversation too:
https://www.joomshaper.com/forum/question/15829
Best Regards