SOLVED!
After installing 1.8 ROCK/NUC performance was much better with less network errors and connection problems. So I decided it was time to replace my old Linksys WRT1900AC router. Luckily there is a shop nearby 
My new router is ASUS RT-AX88U. Firmware updated. Multicast and IGMP Snooping enabled. Firewall is ON. Everything works fine. Setup is exactly same like Linksys (DHCP,DNS etc)
After few hours of stand by SEARCH took few seconds and NOT 4 minutes including TIDAL and local results.
CONCLUSION:
- In certain situations Linksys routers may not be the best option for Roon although the technical explanation of why remains to be a mystery.
- Depending on the connectivity, performance of 1.8 and user feedback, Roon may still require a fine tuning to work in relatively difficult internet infrastructure in the future.
Many thanks to all who contributed!
Enjoy the Music!
Emre