So I contacted DrayTeK UK and DrayTek.com (Taiwan) and they both say basically the same, beta firmware are only released if a particular issue with a firmware is found, they do not make these beta.s publicly available,
I have noticed over the year, random post on Facebook / Reddit where an owner has used a beta and then shared the link.
Below is one of the responses from DrayTeK UK.
Beta firmwares are used to test and improve current and new features. If you need to report a case, please do so directly with our UK based support team. In some cases, a beta can be provided but whenever possible we would recommend official versions.