@deadready 

Do not if you done this or if it will help, but be sure to power cycle the router after the router changes as that messed me up setting up my camera NVR with port forwarding. 

  @ArcherC8 Thanks for the suggestion.  I've power-cycled the router, and have also contacted my ISP for additional assistance.  They couldn't really do anything for me, yet I was informed that they do not restrict any ports, so at least I know they're not getting in the way with opening a port... seems like it's boiling down to the router and it's many features preventing the port from opening.  I've set rules in Windows Defender on the machine designated as the server to allow both incoming & outgoing on the port I'm trying to open.  The ping is successful but the TCP connect fails every time.  I'm also using portchecktool. com to test if the port is open, as well as Windows PowerShell 'test-netconnection <ip> -port:1666' command line.  I'm not sure what else to do at this point. :(

  @terziyski Thank you for your suggestion.  I have not heard of this before. 
Yesterday, my DDNS name did match the resolved IP address, but today it does not. 
I communicated with my ISP last night and they reset my IP, so now the DDNS and public IP does not match. 

So you're saying if they do not match, I'm now behind NAT or CG-NAT?  Any suggestions for how to resolve if that is the issue?

So, to test from outside the network, tell me if my thinking here is correct: disconnect my cellular phone from the network, then use it as a hotspot for my laptop to gain internet access, then try to ping from there? (sorry if that doesn't sound logical)  Thanks again!!