Another TLS / Security question

Hello everyone,

So I just started getting home assistant set up. But I keep failing at the MQTT integration.
I started the broker on my Homey device which also has a self signed secure connection.
I got 3 fields available : Certificate, Private key and public key. I configured home to port 8883.

My router has Upnp disabled and I didn’t forward port 8883 yet. But I can see the MQTT Broker and control all things is this because its in my local network ?

Second question is when I put what’s in the field of certificate in a notepad file and put that on my home assistant (which should be my TLS certificate) home assistant sees the file. But I need to put in TLS insecure : true in order to get things working and im not sure this is ok… ?