Our setup in Japan is as follows:
Japan NTT provided Optical Fibre into our house connected to a Router which is a NTT PR-500Ki with its own plugin Wi-Fi dongle ( NTT SC-4ONE (2) ) The router has a dedicated slot for this wireless dongle.
The router has 4 lan ports of which 3 are used for the 3 switches mentioned ( Buffalo LSW6-GT-5EPL & 2x Netgear GS-308 )
Router is 192.168.1.1 and has been since installation 2 yrs ago. Addressees for all wired and wireless equipment are 192.168.1.xx
NUC Lan cable was attached to the Buffalo switch located in our lounge. Changed cable but NUC still no change of address 192.168.8.xxx.
NUC moved to Netgear switch and address reverted back to 192.168.1.43 so problem solved. So I will replace the Buffalo switch with a Netgear.
Thank all for your hints and help.