Hi @7yuki7 and welcome to the Mattermost forums!
Please see this documentation about how to set up nginx as a reverse proxy for a Mattermost application server:
https://docs.mattermost.com/install/config-proxy-nginx.html
Since you already installed nginx and Let’s Encrypt, etc., you probably will only be interested in the part of the configuration that does the reverse proxy to the Mattermost application server. The example configuration you can see in the docs points to 10.10.10.2:8065 for the Mattermost application server, you should replace that with 192.168.242.182:8065 in your setup.
You need to install the Let’s Encrypt certificate to the nginx proxy then and it will take care of the SSL handling and forward unencrypted traffic to the Mattermost application server then.