I confess to deeply mistrust Weebly, Wix and most online "no knowledge of HTML required" web development tools, but in your case the forum is displaying near perfectly on my 7" Android tablet. The only problem is that the second right hand ads don't appear at all in portrait mode.
My Samsung Galaxy S3 Mini struggles a little more in portrait mode with various parts of the display overlapping and obscured. However in landscape mode it is near perfect with a similar problem to my tablet in portrait mode. The second ad is truncated with no option to scroll to see it.
Using Weebly's "web" link to force the browser to render the page to desktop standards produces nearly perfect rendering albeit with the need to zoom on the smaller screen to make it legible.
Overall, I'd say this is a case where any browser is going to struggle to render a page designed to be displayed on larger screens. In the longer term Nabble and all other forum software providers need to devise a completely different interface for display on small screens. Technically, this is a huge task as Nabble is largely constructed using the HTML table structure, when divisions, lists and other alternatives would be required to design a good interface for small screens, so don't expect any significant improvement soon.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.