If I understand correctly your RUT950 WAN interface is configured using DHCP settings so an IP conflict is unlikely, unless somewhere on the FritzBox LAN there is a device with the same IP address assigned statically. I'd also recommend to check whether the cabling is fine since issues related to packet loss can occur due to faulty cables.
Also, could you please try to isolate the issue by connecting nothing but WAN port to the RUT950 and pinging that IP address from FritzBox LAN side? By default the firewall rules should permit incoming ICMP requests and you should get a reply back - in this case you'd know whether the issue exists directly between FritzBox and RUT950 or, if everything seems fine, the issue only exists specifically when using internet connection.
Additionally, if possible, I'd highly recommend trying to connect to the internet directly from RUT950, without using the FritzBox as the device in the middle.
If these steps don't yield any results, please backup the configuration of RUT950 and try to reset its settings and reconfigure only the WAN interface. If the issue occurs after doing checking everything mentioned here, let me know.