Setting up a second router behind my ISP's router.

I currently have my ISP's router

10.10.10.1 /24

I have a second router I need to install behind it. Its LAN is 10.20.20.1 / 24

I tried setting it up for DHCP (WAN Side) and it doesn't get connectivity.

I tried manually setting up w/ a static IP

GW: 10.10.10.1

IP: 10.10.10.2 (I am 100% sure no other host is using .2)

Subnet: 255.255.255.0

This doesn't work. Devices behind the 2nd router can not access the internet.

But if I set a laptop with that IP info it will work just fine. Can anyone tell me what I'm doing wrong? Thanks in advance.