I had a similar issue which maybe the same.
To diagnose if this is your issue you can do a ping with a set packet size and see if you get fragmentation and 100% loss…
windows command
ping myopenhab.org -f -l 1500
Linux command
ping -s 1500 -M do myopenhab.org
See this link for more info on how to test and set the optimal size…
https://kb.netgear.com/19863/Ping-Test-to-determine-Optimal-MTU-Size-on-Router