Cannot establish secure connection to server - how do I sort this? I’ve tried everything including clearing data/history, resetting network settings etc.

Many VPNs connections, unless your is a corporate one, do more harm than good.

Try uninstalling that VPN app, restart your iPhone, then test your connections again.

If the issue persists, I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.

The server owner you should contact is the company that owns the website.

Can you post a screenshot showing the web address and the error?

What about another browser or desktop?

Restarted the device?

The site is not blocked.

Contact the server owner.

Try from a desktop or another browser.

Does it happen using wifi only, or using cellular data too?

Should be the former, does it only happen at home, or with all different wifi networks?

I am assuming this is one site. Is it all sites?

If so, restart device and try another browser?

Does App Store work? Email?