MTN cheating us - Package changed

I have been using MTN LTE router for about 7 months now, on premium package"500GB - 10mbp and 10mbp if the 500GB is finished, 2 months back MTN made some changes to the subscription now you 2mbp after 500GB usage. But now the problem is that after 300GB they cut the speed to 0.5mbp.

We are no longer getting a full 500GB, and the speed is reduced to 0.4mbp. MTN is cheating us or they have a system error.