Connection to LMS keeps dropping

Hi

Assistance would be greatly appreciated in resolving the issue I'm having with my SBT and LMS... In a nutshell, the SBT works fine, connects to LMS and plays music, but then randomly drops the connection to LMS.

Bit more detail:
- SBT has been running happily on LMS for 5+ years. Migrated to NBN and new router approx a year ago, but no issues
- In the past couple of weeks, the SBT's connection to LMS seems to drop, ie error message "problem connecting" and option to "try again" or "switch library"
- The SBT is connected via Ethernet, so no wireless network foibles
- I've tried (several times) powering down router, SBT, Windows 10 PC on which LMS runs
- I've done a factory reset of the SBT
- I've updated the LMS version from 7.9.3 (?) originally to latest 8.1.1
- LMS is running correctly and showing my music library: I can connect to it and play music with an iPad running Squeezepad and also using the web controller
- I have tried turning off all firewalls and virus - made no difference. In the past it has run fine with Windows Defender and AVG, with exceptions in place for LMS app and ports 9000, 3483 etc

I'm out of ideas and not very tech savvy in the first place!

Much appreciated.
Richard