PHP error (2): fsockopen() Error

The issue I am facing:
Pi Hole is displaying the error shown below. Can somebody in the community please clarify what this error is related to? What does it mean? What needs to be down to fix it?

There was a problem applying your settings.
Debugging information:
PHP error (2): fsockopen(): unable to connect to (Connection refused) in /var/www/html/admin/scripts/pi-hole/php/FTL.php:43

Details about my system:
Raspberry Pi 4B 8GB Ram running CasaOS with Pi-Hole docker installed. I have a C7000v2 Router/Modem combo unit and 9 devices attached. However, for test purposes I only have a Windows10 PC and a Raspberry Pi connected to the C7000v2 Router/Modem.

What I have changed since installing Pi-hole:
Netgear Tech Support says I need to figure out how I can configure Pi-Hole with its own custom IP number, Subnet Mask and Gateway number before I disable the DHCP in Netgear Genie. Otherwise, I will lose internet connection. Is Pi-Hole designed to support such configuration parameters?

I have been getting this error on all 3 of my piholes.

PHP error (2): fsockopen(): unable to connect to (Connection refused) in /var/www/html/admin/scripts/pi-hole/php/FTL.php:43

A restart of the DNS resolver cures it.

That would suggest an issue with the firewall of the machine that runs your Pi-hole, see Pi-hole's required ports.

That's primarily about configuring the OS of the machine that runs your Pi-hole.
If you want to set a static IP address on that device, you should do so through your OS's preferred network management tool (e.g. for RaspberryPi OS, you could check the contents of /etc/dhcpcd.conf whether it would already contain any static definitions and adjust if required).