Fritz boxes have a rate limit on pings which is enabled by default, so you often see failures. Any more than 1 ping a second and they tend to ignore the ping.

Many telcos lock down the settings on the router so you can't change this. There are settings in the config file that you can download from the box, decrypt (google for it) and re-upload, but if the router is being re-configured by the ISP (which they usually seem to do) you'll lose your changes.

You'll have to get your telco or AVM to help you. Or failing that, ditch their router. Having a router under someone else's control at the edge of your network is never a good idea.

they said they will contact think broadband.

Thanks for your ideas/feedback but at the moment i will leave it as-is.

Gabrio