Swapped Modems, same model, static WAN IP, Router couldn't reconnect

No Firewall in Router or in O/S
Static IP addresses for all my devices except for a ATA (Obi202)
Nothing special or fancy in the LAN.
All wired except for a Laptop.

Long story short, I swapped identical Modems that gave a static WAN IP address (yes it was static!) Both had a RF connection, new one showed connected. I moved the Router cable to the new modem with no reboot of the Router.

No connection to the Internet. I rebooted the Router, still nothing. I then noted there was IP assigned. I did a DHCP release and renew twice. Still nothing.

I then reset the Router (which I didn't want to do), still nothing. No WAN IP assigned still.
I changed the LAN start IP address from 100 to 2 since I use static IP address and after a minute the Router finally received a WAN IP address.

Questions;
1. Going from one identical Modem to another that passes an IDENTICAL IP address to the Router, is it still necessary to reboot the Router?
2. Even after rebooting, why wasn't there connectivity between the Modem & Router, even thou the Modem 'Link' LED showed connected and with activity?
3. After the Router reset, is spite of the LAN starting address at 100 which was outside of the static IP's of my devices, why didn't the Router get the WAN address from the Modem??

Win7
Spectrum ISP
Zoom 5341J Modem
TP-Link WDR-3600 Router running dd-wrt firmware