Myopenhab goes online and offline all time

I have a google assistant and openhab running on a raspberry pi (see below). For a long time the system worked fine. I was able to use google to voice control e.g. shutters etc. .
As of lately, which is several weeks now, google assistant is not reaching openhab for most time of the day. I traced it down to myopenhab having no connection to my Pi running openhab 2.5. The events log shows online…offline several times per day. It ends up to be more offline than online. Within the LAN openhab can be accessed and can control all devices. I have the raspberry pi connected to the LAN via power line lan adapter. I swaped it with a different one. The problem is still present, no change. Again from inside the lan network I am not having issues. It is the connection to myopenhab that is getting lost and sometime recovers.

Any idea what to change to prevent myopenhab from going offline?

Platform: raspberry pi 3
openhab: 2.5
connection to internet: power line lan

Kind regards