The web is turning https.
The web is gradually turning to using https rather than http. Chrome eventually will only support https. https on chrome I am not taking a political side on this here, just stating what is happening.
I made my DS213j serve up https rather than http if possible, as this seems to be future. However when I check my website it was marked as insecure. Digging about it turns out that the https needs signing with a valid certificate rather than using synology’s default one.
The problem is solved by installing a certificate from lets encrypt. This is a non-profit organisation aiming to forward https across the web. The synology app apparently reinstalls a new one every 3 months before it expires.
Use lets encrypt for free web certificates.