Your mobile menu isn’t getting generated at all which is why it doesn’t show. A workaround that would only work for you is to just show the regular menu on mobile since your menu is only a few items.
.js_active .main_menu ul {
display: block !important;
}
The actual issue however is either a plugin or alteration to the main js file for the theme though I can’t say which just from looking at things now.
Where should I be placing the code, I tried in the child theme and parent and still same results