This explains the 403.
https://www.hostinger.com/tutorials/what-is-403-forbidden-error-and-how-to-fix-it
Even though it's an app, it has to communicate with the back end hosting servers. That would be done over HTTPS. You would have to run a network trace to capture the HTTPS address being rejected.
For the most part, you can't "fix it", per se. There most likely is something in your network configuration that is interfering with the traffic. Or just a simple bug in their app.
Are you using a VPN? Are you accessing the internet via a proxy (typically in a business or school network)? Are you on a home network with a direct connection? PC-->WiFi router--->Internet? If you are using a laptop, go visit a friend and try using their WiFi connection to the internet.
Can you use betonline's website instead of the app?