I saw a similar report here but there is no solution:
https://www.joomla-monster.com/joomla-monster-forum/free-templates/templates-for-joomla-3/jm-services/menu-doesnt-show-up-on-mobile-version
I installed JM Services - Free Joomla Template and noticed there is an important bug.
The main menu shows up correctly in my PC Chrome/Firefox browsers and iPad Air in both landscape/portrait mode.
But some menu items load/show up for a split second and then disappear immediately in extra small device mode (in my case I tested with both iPhone XS and XR) only when the phones are physically turned into PORTRAIT mode (show up correctly when turned back to landscape mode).
Test site: kltga.my (offline access with login: test/test . The login form is located under the CONTACT menu, top-right corner.)
I have also installed a FRESH copy (without changing anything) of the template at:
Test site: langhoose.com/joomla (online & no login needed)
After some trial and error, I noticed that the menu items will show up or disappear based on the MENU ITEM TYPE selected. In my case, if I select CATEGORY BLOG or JEVENT > VIEW BY MONTH, the menu will show up correctly under these two pages.
I really appreciate some help here. I have working on this for a week. I only noticed this bug just before I handover the website to my client.