Hotspot devices not forwarding to external login page.

I have a hotspot setup in a hotel. For the most part it works perfectly. However, about one phone in 20 cannot sign in. I am using the standard login.html for diverting form the Mikrotik website, to forward to an IIS ASP webpage where we undertake the actual sign in.

As I wrote for 19 out of 20 phones all works exactly as I expect, but for the one it remains with the “If you are not redirected in a few seconds, click ‘continue’ below” message. But clicking on “Continue” does nothing.

I thought maybe it was because JavaScript is disabled on these phones, but then I would expect the Continue link to work as this is not JavaScript.

The page it is supposed to forward to is working, and the IP address is in the walled garden settings, and we forward using IP address not DNS, so this is not a DNS issue.

Has anyone seen an issue like this before? Is there a remedy? At the moment this is upsetting far too many guests, and hence causing me a headache as I have to manually setup up a bypass for their phones.