Thank you so much, Bigfoot65. My HDA is back on track. Everything works as expected.
Just for the record :
1- I set up my router's DHCP settings "Start IP Address" and "End IP Address" both to the same address, matching the HDA "original" IP recorded in the control panel.
2- rebooted router & then turned the router OFF (to be sure)
4- turned OFF the server/HDA & all clients (Wi-Fi + wired)
5- then turned the router back ON (waited a bit to make sure everything was correctly initialized)
6- turned the server/HDA ON
7- in the HDA, entered the
command in a terminal to make sure that local hostnames were resolving properly (and it worked!)
8- then turned clients back ON one by one (verifying that each had LAN and internet connection)
... & now the mobile Amahi app works as expected (love it), and my good old server is my friend again.
Again : thanks Bigfoot65 and Carlos for your patience, generosity and intelligence. I know the Fedora version is the new great thing, but... I like my old Ubuntu : it works well and does what it's supposed to do. Maybe in a year or so...
What I Iike with Amahi is of course the fact that it plays well with other OS & file systems. But the people behind it really make it shine.