I believe to have found a work-around for this issue. Apparently, I’m not the only one suffering as, the Raspberry Pi4B has issues like this in other instances.
Since my FBX is wired to a router and my router does all the port forwarding, I found it safe to continue with the below. If you are directly connected to the internet or connected with DMZ, please do not use this.
I followed a path of masquerading my Internal network and configuring my wired connection as Internal. At the cost of practically not having an Internal and External network any more, my problem disappeared.
If you wish to take this route, you may apply the following:
firewall-cmd --zone=internal --add-masquerade
firewall-cmd --zone=internal --add-masquerade --permanent
Next, use the Plinth interface to go to System > Networks and change the setting of your WAN connection to Internal.
Now, as a warning I must admit I do not have knowledge to the full extent of implications for the above. I am depending on my router to provide my security so if anyone wishes to comment on this resolution, please feel free to do so.
Thanks.