Thanks to all who offered help. I needed to purchase a dedicated IP address from my ISP. That solved the problems. Raised my recurring monthly charges, too.
Julian, that sounds like you issue was cgNAT by your ISP.
a dedicated (typically static) IP is required to avoid it.
I really don’t like when ISPs do that, sometimes their tech support don’t even know of this practice which is even more annoying.
Hope you got your issue sorted out.
I would consider moving to another ISP that doesn’t use cgNAT if I were you.
I had the same issue with mine and was about to move to another iSP before my current ISP agreed to give me the dedicated (static) IP for almost free.