I am using lxle 18.04 i386 on old laptop 2009ish
Situation:
new install running fine seamonkey plus internet access okay
I had a reason to set the IP address to a fixed low number
I carried out CL tasked I wished on a embedded system over ethernet but not on original network
I put the ethernet(network) on the PC back to DHCP(automatic) reconnected to original network
The network information tab after disconnection and reconnection shows the DHCP address( not the fixed low number) and modem gateway.
The apps (seamonkey etc) cann't access internet websites saying "access denied"
This continues even after a hard reset /power down reboot.
WHY!! what point am I missing and how can I correct it.
Help Please.
I know Im being a donkey but I am after all a hardware Engineer

Welcome to LQ! I wanted to toss out a quick Hello & (gentle friendly) advice.

Web search: [basics of beginning] linux net troubleshooting
to see what details will be needed, to see the state of the device, and my fav:
That will guide you to collecting essential info for troubleshooting.
Then lots of others can help here. Best wishes!

Thank Ill give it a try

I re-read your details more closely, and realized:
[and web searched] ubuntu 18 set fixed ip
has gotten trickier on Ubu derivites: netplan (YAML language! )
Which may be why 'it' (whatever you did) didn't 'simply' work.

https://linuxize.com/post/how-to-con...n-ubuntu-18-04

I'm not sure what you meant by 'CL': "I carried out CL tasked I wished on a embedded system over ethernet but not on original network" (is there a second NIC?)

Oh, when you resolv it, post some info on how&what, so others can find the sol'n in future web searches. (The trick is to have just the right keywords to uniquely specify the precise scenario. Like: how [exactly] to reproduce the problem: if others can reproduce it, they can explain/fix it!) Then there's the "Solv'd" button, on top of post#1.

Let us know...

I used the network edit app to set a fixed ip.
then used the command line CL to ftp an image into an embedded system before reversing the process with the network edit app . The network app is the one that is accessed via the network icon on the screen. I unplugged the rest of the network so the PC and embedded system stood alone. When finished reversed the process and reconnected to the main network. The PC and embedded system connected via a network switch to which the external network was removed and then reinstated.
As the app appears to be integral to the system my assumption is that it deals with rewriting the appropriate files and restarting daemons etc as required.

My first guess is that the fixed IP you set may not be in the subnet established by your router, so that the machine is therefore unable to connect through the router to the Big Wide World.

What response do you get when you try to traceroute an external address? (This will tell you where the connection is breaking down.)

Can you ping or traceroute the router itself?