The german ISP Vodafone / previously Kabel Deutschland are using a branded router / modem labeled "vodafonestation". Previously, for example Hitron devices were used.
The "vodafonestation" can be placed in bridged mode, unfortunately it does not behave like a bridge, at the very least not in my case. The "firewall" is still showing up as turned on, and all nontranslatable protocols (f.e. 41, 47, 50, 51) seems to be silently dropped.
Unfortunately, vodafone germany is one of those ISPs whose support staff have limited interest in solving or identifying such issues.
I thought I should drop a line here, perhaps it saves someone work & energy.
Business Service, which is cheaper and dualstack and possible also includes service, is available from the same ISP, but only if you have a registered business (Gewerberegisterauszug).