Hi @grandprix !
It would greatly help if you could be so kind to share more details about the current configuration that works with speed-limit set to 20 Mbps and how exactly did you try to raise the limit to 50 Mbps. Also, I couldn't understand if the link that you are trying to "increase" can handle the desired bandwidth at all. Did you try to remove all the limits and see if that link is capable of 50 Mbps?
Another point - what is that misterious "rate-limit" command that "doesn't work"? You can set the rate limit in Comware 7 either by QoS policy or directly on the interface by "qos lr ..." or "qos car ..."" commands, but I am not aware about any "rate-limit" command you are referring to...