Website being blocked by O2

[NicCro]

My website https://cottonliveson.org is being blocked by O2 but not by any other mobile provider. 

I have tried livechat and calling 0344 8090202 but have not managed to speak to anyone who might be able to help. I did see that there was an O2 url checker, but I can't access that either. 

Any help in how to get this solved would be appreciated. 
Thank you 

[Enlli]

Working here

[NicCro]

Strange. Are you using a mobile device?
Myself and my client (plus a few others) that use O2 for mobile data can view using wifi but a soon as wifi is switched off, we all get an error message, which others using different mobile data providers don't? 

[Enlli]

S22+ Mobile running 5G Liverpool City Centre

[NicCro]

The message that we are all seeing is 'Safari cannot open the page because it could not establish a secure connection to the server'. I know at least 4 of us are O2 via Tesco, not sure about the others. 

It's the only website I have come across that I can't see with wifi turned off. 

[MI5]

Working for me too.

O2 4G near Coventry

[Enlli]

Notice you mention Safari. 

Using Chrome here. I think @MI5 maybe too. 

If so it's a browser issue.

[NicCro]

On Chrome we get a similar error - 'This site can't provide a secure connection - sent an invalid response. ERR_SSL_PROTOCOL_ERROR

Hosting company has checked the SSL certificate and the site works fine on all browsers on PC and phone with wifi on - just not when wifi off. Its strange that you can see it though with wifi off. 

[MI5]

Must be a mast issue at the location you are using @NicCro 

Working elsewhere fine as attested to by myself and @Enlli 

[madasaf1sh]

Nothing to do with the mast, as the Mast doesnt provide the Internet connection, it provides a connection to the POP (Point of Presence), 

What happens when you use Edge, Safari or Firefox or any of the variants of those?