New internet provider using existing netgear router

What model router do you have? 

What is the Mfr and model# of the Internet Service Providers modem/ONT the NG router is connected too?

This would be a double NAT (two router) condition which isn't recommended. This would be a double NAT condition which isn't recommended. https://kb.netgear.com/30186/What-is-Double-NAT
https://kb.netgear.com/30187/How-to-fix-issues-with-Double-NAT
Couple of options,
1. Configure the modem for transparent bridge or modem only mode. Then use the NG router in router mode. You'll need to contact the ISP for help and information in regards to the modem being bridged correctly.
2. If you can't bridge the modem, disable ALL wifi radios on the modem, configure the modems DMZ/ExposedHost or IP Pass-Through for the IP address the NG router gets from the modem. https://kb.netgear.com/25891/DMZ-on-NETGEAR-routers
https://kb.netgear.com/24086/How-do-I-set-up-a-default-DMZ-server-on-my-Nighthawk-router
3. Or disable all wifi radios on the modem and connect the NG router to the modem, LAN to LAN configure AP mode on the NG router.
https://kb.netgear.com/24104/How-do-I-change-my-Nighthawk-router-to-AP-mode-after-I-ve-already-run-setup
https://kb.netgear.com/26765/Disabled-Features-on-the-Router-when-set-to-AP-Mode

Try option #2 first...

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thephilnewman wrote:
I have just changed internet service providers and I want to use my existing router but don’t know how to set it up

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In all of the complicated stuff in the other replies to you, I can find no mention of one essential bit of advice.

You should get your existing router to forget any settings it picked up from your old ISP. So you need to get back to the initial settings.

How do I perform a factory reset on my NETGEAR router? | Answer | NETGEAR Support