The “Unable to setup forwarding” error is happening because by default, Ubuntu doesn’t allow processes to bind to ports lower than 1024. You’ll need to give mattermost the “cap_net_bind_service” capability as described here: https://docs.mattermost.com/install/config-tls-mattermost.html
As for the “server could not prove that it is mattermost.toradex.int” error, I’m guessing yes, you probably need to add additional parameters to the certificate. Perhaps this stack overflow answer will work for you? https://stackoverflow.com/a/43665244