Further - it went wrong again this evening so it was time to experiment. I plugged in the ethernet cable on the Mint laptop and gave it a few seconds to connect and then the internet access was restored. It still showed the wireless link to be working in the network manager but only the wired connection gave any WWW access. Also I noted that a Win10 laptop carried on working wirelessly throughout the testing with the cable etc.
So, in summary, the Mint laptop loses the wireless - router/modem - WWW connection but the Win10 laptop and an android phone may carry on accessing the WWW or may not. Getting the Mint laptop back on line wirelessly requires a reset of the router/modem. Rebooting the Mint machine, resetting the network manager or just relaunching the browser does not stop "Server not found" timeouts on all websites. The admin logging doesn't work. If I plug in a cable to the router WWW access is fixed but when unplugged the wireless access is still broken.
The new router does not have a 0000 default admin password, it has a 8 character alpha numeric with upper case and lower case. It looks as though it fetches this password when it logs onto the ISP or it might be "hard wired". I have not found anywhere in the admin login menus that suggests a password change. This password is proper printed on the pull out plastic card in the slot on the back of the router and is still valid after a factory "pin in the hole" reset. Ping 8.8.8.8 only works while the cable is plugged in if the wireless to WWW is broken.