I see, and I traced already: yes, NFS is sending DNS queries to external server at configuring 'tango3'. Of course that server has no clue about 'tango3'.
Yes, I keep reservation for a bout a week for each user in my router. But when returning back from vacation: need to reconfigure again. I will change to static IP on client side as you suggested because this seems to be easier than configuring DDNS or something like that.