Consider your current network needs and performance to decide.
Consider your current network needs and performance to decide.
Can you swap in a new gateway and link it to your ONT, or should I simply swap out the existing WiFi extender?
By default, connecting it to your ONT would make it your main gateway. Switching to AP mode would simply use your current gateway for WiFi. Replacing the gateway might not offer much advantage unless you need better performance or features. Wired speeds appear to work well based on what you've noticed.
It seems a fresh router likely includes additional capabilities making it the preferred choice by default.