DHCP trouble with Kabeldeutschland (Vodafone) - DOCSIS3


have some trouble switching from Fritz.Box to OpenWRT 19.7.6 as DHCP does not work fine with my cable router in bridge mode.

I enabled DOCSIS3 support and normally get an external IP. But sometimes I did not get a successful DHCP request trough and have to restart my cable modem. In the post below I found a remark that DOCSIS3 will deliver IPs in a short time after restart of the modem only. But I never had issues with my Fritz.Box but it matches what I see. Mostly I have to restart the modem if I have to restart the router. If both works fine I see no problems but actually installing some packages I run into that. Is there a way around this problem or is it a must to restart both devices? I also want to stay in bridgemode.