OK,
Updated firmware and added IgnoreCaptivePortal line in .conf file on kobo ClaraHD and true enough kobo is happy to join intranet network.
Also did the same update for kobo mini, but unfortunately it didn't make any difference.Maybe it was because I was unable to connect to kobo account?
Plus some of the screens got a bit messy (new firmware? or just an old mini?)
All above was while I was connected to kobo account, however...
When I tried the same while not connected to kobo account (I use kobo for gliding) things didn't work and I still had to pair kobo first with hotspot on the smartphone before it now connects happily to my intranet device.
Any attempts to join the device directly failed, despite the fact that kobo was doing it happily while connected to kobo account
Sounds like I will have to ask more questions on gliding software forum.
all who have patiently helped me.