The issue on your website is due to your advertisements being too wide for mobile devices, and they are destroying the device's viewport window.
UBB.threads 7.6+ was written to allow a minimum of 320px viewport.
Your header advertisements are 512px wide.
When I test using an iPhone XR, your page pushes the screen out past the iPhone XR's 414px viewport width.
Viewport Sizes iPhone414 x 896 --- iPhone XR
375 x 812 --- iPhone XS
414 x 896 --- iPhone XS Max
375 x 812 --- iPhone X
414 x 736 --- iPhone 8 Plus
375 x 667 --- iPhone 8
414 x 736 --- iPhone 7 Plus
375 x 667 --- iPhone 7
414 x 736 --- iPhone 6 Plus/6S Plus
375 x 667 --- iPhone 6/6S
320 x 568 --- iPhone 5
iPod320 x 568 --- iPod Touch
iPad1024 x 1366 --- iPad Pro
768 x 1024 --- iPad Third & Fourth Generation
768 x 1024 --- iPad Air 1 & 2
768 x 1024 --- iPad Mini 2 & 3
768 x 1024 --- iPad Mini
A few viewport sizes from popular Android devices:
Android Phones
412 x 732 --- Nexus 6P
412 x 732 --- Nexus 5X
412 x 847 --- Google Pixel 3 XL
412 x 824 --- Google Pixel 3
412 x 732 --- Google Pixel 2 XL
412 x 732 --- Google Pixel XL
412 x 732 --- Google Pixel
360 x 740 --- Samsung Galaxy Note 9
480 x 853 --- Samsung Galaxy Note 5
480 x 853 --- LG G5
480 x 853 --- One Plus 3
360 x 740 --- Samsung Galaxy S9+
360 x 740 --- Samsung Galaxy S9
360 x 740 --- Samsung Galaxy S8+
360 x 740 --- Samsung Galaxy S8
360 x 640 --- Samsung Galaxy S7 Edge
360 x 640 --- Samsung Galaxy S7
Tablets
768 x 1024 --- Nexus 9
600 x 960 --- Nexus 7 (2013)
800 x 1280 --- Samsung Galaxy Tab 10
1280 x 850 --- Chromebook Pixel
this data was sourced from:
https://mediag.com/blog/popular-screen-resolutions-designing-for-all/