You can verify if remote access is working via websites such as canyouseeme.org.
Go to the site from a system on the same network as your Plex server.
Use your remote access port instead of the default, 80.
It will tell you if it can see your Plex server.
You can use Uptime Robot to continuously monitor remote access. It is similar to canyouseeme.org, checking your remote access port every five minutes. It will send an e-mail if it cannot reach your server (only one, not one every five minutes). The free account is sufficient to monitor a Plex server.
You can monitor playback via Plex Dashboard → Now Playing. Connections listed as Remote are using remote access. Connections listed as Indirect are using Plex Relay.
I am having the exact same issue as of last week. I turned off the relay but it still slows me down to 2mbps. Everyone across the board that I’m sharing with has been having issues with quality of streams since this has broken down.