Internet Works then Doesn't - Is it Local or is it ISP?

A little background:

ISP Modem setup by ISP tech. WAN port on modem connected from modem to WAN port on firewall. LAN port on firewall flows into managed L3 switch, goes out to devices.

ISP Provides following for WAN interface port:

- Static IP Address

- Subnet

- Gateway

- DNS Server #1

- DNS Server #2

== INTERNET WORKS - LIFE IS GOOD

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Current Situation:

ISP comes back, installs new hardware (modem box). Plugs WAN port on modem to WAN port on firewall. LAN port on firewall flows into managed L3 switch, goes out to devices.

ISP Provides following for WAN interface port:

- Static IP Address

- Subnet

- Gateway

- DNS Server #1

- DNS Server #2

== EXACT.....SAME.....SETTINGS....NOTHING....CHANGED....

Using Static IP address, with configuration on firewall, it would not resolve DNS = no internet. Only after re-configuring WAN interface port to use DHCP from ISP to grab an IP does it talk to DNS correctly, resulting in Internet.

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My question is, how if the same ISP is used, with the same ISP settings as before, does the firewall fail to resolve using the exact same Static IP? The only thing that is different is a new modem box. Is it a local issue, or is an ISP issue not having the static IP routable internally through their end?

Love to hear your thoughts on this. Many thanks for those that respond. Harsh/snark responses not appreciated.