HassIO, Letsencrypt and local SSL - certificate error

Hi guys,

how are you dealing with the issue, that when I activate DuckDNS (to use Google Home, etc) I always get an “certificate error” on my local network.
I am trying to set up a wall panel in kiosk-mode that accesses HA through the local hostname, but it fails thanks to the encryption problem.
Accessing HA through the internet all the time is somehow not a solution for me, since I want to be able to switch things on and off even when the internet is down.

Any ideas?