@Peter_Weber 

Hi,

Beside the IPTV, do you have more special features activated? Like Antivirus, Parental Controls, QoS, etc.?

Basically, any feature that requires data packets from the Internet to be analyzed has the potential to reduce the data throughput, because it increases the work the CPU has to do.

  @woozle only fire wall is active. Antivirus, Qos, parental control are OFF.

  @Peter_Weber 

Sometimes it's not so obvious which features might affect the data throughput through the WAN port.

If you want to know if 408 Mbps is the limit for the AX50's WAN port even with default settings, then you could do the following test.

1) In the admin webpage go to "Advanced" -> "System Tools" -> "Backup & Restore" and make a Backup of your current router settings.
2) Perform a full "Factory Restore" from the same menu as above.
3) After the router has rebooted, go to the "Internet" menu and make only the changes necessary to connect you to the Internet. Don't change any other settings on the router and don't log in with a TP-Link ID !
4) Perform another speedtest_net like you did before and see whether the numbers are now any higher or still the same as before.

To bring the router back to the original state just "Restore" the previously saved settings and reboot.

Depending on the result of step 4 we may then investigate further.