GoKawiilInternet providers often supply a gateway, combining the modem or fiber connection with router functions and Wi-Fi in one box. Connecting your router to that gateway doesn't automatically stop the ISP equipment from routing traffic, so leaving both devices in their default modes can produce double NAT.
If you want your own router or mesh system to control the network, check whether the ISP gateway supports bridge mode. Bridge mode stops the gateway from acting as a router, leaving your own router to manage the network instead. That also avoids double NAT. The exact procedure varies by provider and hardware, so this isn't a setting to change from generic instructions. Check the ISP's documentation first, particularly because bridge mode can affect other functions supplied by its gateway.
The other option is to keep the ISP gateway as your main router and put your own router or mesh system into access point mode. In that setup, the ISP gateway still handles routing, while your secondary device mainly provides Wi-Fi coverage. The downside is that some router-specific features on your device may stop working, which is a particular pain since many routers offer more features than ISP-provided models. TP-Link, for example, says AP mode disables functions such as NAT forwarding, VPN, QoS and some parental controls.
The decision depends on which device you want to manage the network. If you bought a new router specifically for its routing features, bridging the ISP gateway makes more sense. If you're satisfied with the gateway and just want better wireless coverage, AP mode is the simpler arrangement.