Welcome to the forum!
First, let's go through the steps necessary for accessing MediaBrowser from an Internet connected device. First, as you noted you must have the correct port forwarded in your router firewall. For MediaBrowser, the port is 23424, although you can also use port 23524 when using SSL. However, setting up SSL to work correctly is a task unto itself, so I have stuck with only using 23424. You forward that port from the Internet connection to the IP address if your Serviio computer. It's important to use a static IP address on your Serviio computer, as you need that port forwarding to be pointing to the right address.
Now, from the Internet side of the connection, you need to have an address to reach your home network. You can't use your public IP address, because unless you pay your ISP for a static IP, it's going to be changing constantly. So what you need is a dynamic DNS service that provides you with a web address (URL) that you just type into the browser. There are many DDNS services available, and some even provide the service for free. I use a free service called DNSExit, but there are plenty of others. You usually have to run an app on a computer on your home network that periodically checks your public IP address and sends an update to the free service with the new IP address. This way, you always use the same web address to get to your MediaBrowser page. I just checked my public IP address, and I see my address isn't up to date, which means I can't get to MediaBrowser myself from the Internet. I just fixed it.
I see they no longer use an app, and I had to add a script to my computer and run it every 15 minutes to check for any public IP address change.
Finally, once everything is in place, you need to enter the correct web address into the browser. So, let's assume my DDNS public name is atc98092.homeip.net (it's not, just an example). In the web browser, I would enter
https://atc98092.homeip.net:23424/mediabrowser. Notice I have the port number specified in the web address, as well as mediabrowser after the slant. You need the complete web address, including the port and the /mediabrowser at the end.
Dan
LG NANO85 4K TV, Samsung JU7100 4K TV, Sony BDP-S3500, Sharp 4K Roku TV, Insignia Roku TV, Roku Ultra, Premiere and Stick, Nvidia Shield, Yamaha RX-V583 AVR.
Primary server: Intel i5-6400, 16 gig ram, Windows 10 Pro, 22 TB hard drive space | Test server Windows 10 Pro, AMD Phenom II X4 965, 8 gig ram
HOWTO: Enable debug logging HOWTO: Identify media file contents