There are only two options to work with desktop/mobile devices in Mediawiki as of 1.37.
Either use tags <span class="nomobile">hide this</span> or <div class="nomobile">hide this</div> for desktop. And similarly <span class="mobileonly">hide this</span> or <div class="mobileonly">hide this</div> for mobile devices. This will work in templates but the code will only be hidden, still downloaded completely to both desktop and mobile including any (huge) images.
Or use the extension:MobileDetect which is still compatible with 1.37 but it has been in Beta status for four years which means it can become Unmaintained in the future. It will not work in templates only in the pages but only the mobile/desktop part will be downloaded.
Your choice: between rock and the hard place. MobileFrontend has been around for 8 years, magic word indicating mobile access could have been part of the core.