U may want to resonsider the following :
a. Verify with your ISP that the actual switching has taken place for the 1 Mbps service
b. Does your ZTE thingy hv its own inbuilt router ? Not always, bt it has been observed that certain NAT devices (more of a individual unit Hardware/ Firmware issue) do let you down on speeds. You may want to ensure it is operating in the 'Bridged' mode of operation.
c. How are you measuring the speeds ? Over P2P ? in which case it may not be giving you a fair idea, so happens .... is more reliable on some days than on some others. Try downloading over different websites/ portals since certain websites may have this bandwidth limitation imposed on multiple downloads by the same user/ account.
Importantly, have this verified across downloads of various sizes so as to provide a stable, statistical n reliable measure of the occurence
d. Verify whether you are hosting any service .... FTP, WWW etc. Essentially, though the Upload and Download channels are different and no interference should occur, I had this happen to me on more than a single occasion where increased Uploading would render my system slow and consequently unable to take the increased Download channel bottleneck on offer. This may be rare but is nonetheless existent. A variation of this may cause your ZTE unit to respond slowly/ hangup, replacing the same should help you determine the cause as well.
e. Finally, try getting multiple reports of online speed tests from dslreports.com etc
