I'm having an issue with a website that I built where when it is loaded over someones's virginmedia wifi connection, an SSL error is shown no matter what browser is used. However when viewed on their same device but over a 3g network the site it show fine. The website is encrypted using letsencrypt and hosted on a digitalocean droplet. I don't know too much about encryption so I'm struggling to work out whether this is an issue with the SSL config or the person's internet provider.
Any insight into this issue is greatly appreciated.