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.
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Hello xxxx
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.
https://www.draytek.co.uk/support/contact-support (UK/Ireland Only)
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In a completely sane world, madness is the only freedom (J.G.Ballard).