Really not sure how to set this up, all the guides are overwhelming and delve into VPN and all sorts of other seemingly unrelated content.

I have got as far as generating a self signed certificate and when I visit the Https admin page of the router, my browser tells me it's untrusted and I should continue.

What next? 

Is there a way to use use let's encrypt or something similar?  It's not this difficult on my Synology nas, which takes care of this, along with cert renewals.

I gave up, you need a trusted certificate externally signed. There are guides out there on how to get one but too many steps for me so sticking to http.

https://www.draytek.co.uk/support/guides/kb-local-certificate-management