I have just replaced a 2860n with a 2866ax...
I have imported a Lets Encrypt SSL certificate successfully into the 2866ax, however I do not appear to have an option anywhere to change from the default (self signed) certificate to the new certificate.
This used to be accomplished in VPN & Remote Access\SSL general Setup - However, there is no drop down to change certificates - It just lists "Server Certificate Defualt Certificate" (sic) with no drop down to change to the Lets Encrypt cert.
All the doco appears to be quite old but also point to this same place to change - https://www.draytek.com/support/knowledge-base/5135
Does anyone have any ideas how to resolve this? i am running the latest 4.3.2.1 firmware

When you say you imported a Let's Encrypt certificate, do you mean you obtained one via the DrayDDNS system - https://www.draytek.co.uk/support/guides/kb-letsencrypt ?

If you do it that way I believe you can change to it in the SSL set up; but I also seem to recall there was an option to use the Let's Encrypt certificate for all sorts of things on my 2927ac a few firmwares back, which no longer appears in my 2927ax.

I’ve just migrated from a 2862 to a 2927 running 4.4.0 and the setting for using an imported Lets Encrypt certificate has changed. Import the certificate as before but then go to “Certificate Management”/”Local Services List” and select the new certificate in the drop-down box.

Thanks both for the responses. The 4.3.2.1 firmware does not have the option for "Local services List" under Certificate Management. I tried installing the 4.4.0 firmware from the draytek.com site - but this failed to install and corrupted the firmware twice! So I have reinstalled 4.3.2.1_BT and raised a support case.
The online demo for the 2866Lac has the drop down where I would expect it, so I think this is a bug...

4.4.0 RC6 firmware received from support this morning and this has resolved the issue.
Special thanks to @BookIT as the setting has indeed now moved to Certificate Management/Local Services List - Support didn't mention this!!