Access HA via phone on https error

Having a strange problem with HA. I set up https using the following guide: Installing TLS/SSL using Let's Encrypt - Community Guides - Home Assistant Community

Now if I visit https://myhome.duckdns.org on my local network then it works every time.

However, if I visit it on my phone (external to my local network), it loads once, but if I refresh the page then it stops working. Curiously if I restart my router then it loads again, but upon refreshing I just get certificate error:

NET::ERR_CERT_AUTHORITY_INVALID.

It asks if I want to continue. If I proceed on from this screen I get the error:

Forbidden. You don’t have permission to access /UI on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request

Anyone have a clue what’s going on?

OK I figured it out - Leaving this here in case anyone else runs into this problem:
I have a WD NAS which was configured to be accessible to the outside world - I must have set that up yonks ago. This auto configured my router to route 443 to the WD drive (although this wasn’t apparent on the router settings). So after restarting my router I was able to access HA, but after a couple of mins once the WD NAS had reconfigured my router it stopped my access. I was getting the ERR_CERT_AUTHORITY_INVALID error because after the NAS had reconfigured my router it was loading a WD cert.
Needless to say I turned off the ability to access the WD from the outside world and everything started working properly. Took so long to figure that all out!