The following steps worked for me to open my app within de the Mendix Native App and open my app in the browser when I fill in my IPv4 adres + :8080 (didn't have to disable my whole firewall):
First, the steps that Mendix recommends:
1. Be on the same network as your mobile device
2. On your computer with WiFi, set properties to "Private" instead of "Public"
3. In project settings under your 8080 server tick "open to other machines"
If it still doesn't work after this, which it was the case for me, you can try this:
1. To firewall & Network security
2. To advanced settings
3. Set OpenJDK Platform Binary to 'allow connection' and 'profile' = 'all':
Double click on the line -> check 'allow connection' -> go to tab 'advanced' and check all 3 profiles. See the photo below for an example of how the settings should look. (if needed, repeat this steps with the 'Mendix Native Mobile' line)
4. Check if it is working now. If not, you can still try the following steps.
5. Within the tab 'Rules for incoming connections' right click on the tab and then 'New rule'
6. Check the line type 'Port'
7. Choose TCP and fill in at 'specific local ports': 8080 (port 8083 may also be useful to allow, this uses Mendix in some cases)
8. Now it should work!