There is no way to display the secondary menu on the mobile terminal

Thank you.

Hm, weird, looks like you have JS errors which prevent it from showing?

Could you press F12, and screenshot if any errors? They will block other scripts from working.

FYI, the following rules obviously override the previous one that you removed:
https://git.drupalcode.org/project/ultimenu/-/blob/8.x-2.10/css/ultimenu...

Aside we no longer use display:none so to have proper transitions which are not possible with it.

Could you double-check it, and screenshot it?

Our site uses different display methods and there are some conflicts with the current ultimenu plugin, I just solve it for our site.

Thank you for your answer

As said in #3, looks like you are mixing SASS or LESS with plain CSS, more details:
https://stackoverflow.com/questions/21323818

Even if it works on your end, it would not be valid. Might just break other things.
Just an FYI :)

2.x was retired.
3.x was out with some mobile fixes.

Feel free to re-open if still an issue. Be sure to mark it 3.x.

Thank you.