@Alexandre. Thank you very much for your advice. As you thought the existing Deco units did not downgrade. The 'new' unit was successfully updated via the Upgrade Tool to Firmware Version 1.4.0....

Today I added a new Deco unit to my existing network. The existing 3 units are all on Firmware Version 1.6.2 Build 20230906 Rel. 80158. From what I read here units running this outdated Firmware...