Hello, friend!
The HG8145V5 is designed as a home gateway - it doesn't support the function to be a WiFi extender.
If you create a bridge WAN interface to bind the LAN and WiFi to the WAN interface, it may work as one AP, but not sure if it can work with no problem.
If you need an extender, you can use the latest new dual-frequency WiFi ONT with the specified AP to extend your WiFi. For example WA8011Y.
Thanks!