I have a TV-box from a cableproveider, wich comes with an app to watch TV directly on the phone, but only if the android-phone is connected through wi-fi on the same LAN as the TV-box. I am hoping there is a way to make my phone "beleive" it is connected to that exact LAN when i am logged on to another network. Even if it is in another country. Is that possible, and if so, how? My router is a ASUS RT-AC66U.
Usually this should be possible, however it can get a bit complicated:
Check your Internet provider if you really get your own IPv4 IP address when connected to the Internet. If you are sharing the IP address with other people you can just give-up here.
Set-up a VPN server in your LAN (an old PC or a Raspberry). There are a lot of potentials servers and configurations. Make sure you choose a secure on and protect your account with a strong password.
On your router forward the VPN port to the Internet. Warning: If you set-up the VPN not well secured anybody on the Internet can connect to your home network and connect to all your devices that are usually not visible from the Internet
When not at home you must be able to find your VPN server. Usually you use a dyn-dns service. On the computer running the VPN service ayou can also install the dyn-dns client. It updates the dyn dns record in case your public IP changes.
Configure your Android device to connect to your VPN server. After the connection has been established the Android device is visible in your home network and video streaming should be possible.