Hi
Maybe someone have a knowledge to the following issue/problem.
When the router is not connected to Synology/Home assistant I am able to login to the router using 192.168.0.1.
But -
As soon as connect the Synology/Home assistant I am not able to login to the router. Hereafter I must disconnect the Synology/Home assistant and restart the router.
If it is a Sagemcom, Yousee (internet provider), Synology or Home assistant issue I do not know.
Too much info is missing here, but my first guess would be IP conflict: some device has same IP as your router, so check IP of all devices.
If it was an IP address conflict with the router, then you would normally lose internet access too.
Some routers are not good with portforwarding/port opening on the same port as the one used for administration. A normal portforward/port opening would still be there when HA is down, but an uPNP opened port could make sense then.
It could also be that you have added a monitoring integration for your router to HA and your router can only handle one administrative connection at a time.