There are a couple of causes that can be the reason for a slowdown:
1) The device has been moved and the cell signal simply isn't as good at the new location.
2) The monthly data limit has been reached (Which would cause speeds to drop to sub-1-Mbps download speeds)
3) The device needs to be power cycled to update with tower changes.
4) The account has been flagged for abuse.
First thing to try:
The tower companies constantly do upgrades to their towers which occasionally means the devices connected to them have to be power cycled to clear the towers cache of old information.
To do this, unplug your device and leave it unplugged.
If you have a device that the SIM card is easily accessible, remove the SIM card. DO NOT PULL ON THE SIM. You want to slightly push it in and you will hear a click and then the card ejects, you can then pull it out.
Wait 5 minutes with the SIM out and the power unplugged.
Reinsert the SIM card. You will hear another click as the SIM catches.
Plug the unit back in and give it up to 10 minutes to reconnect.
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If this does not help with the speeds, please start a support ticket (or call) and we can diagnose what may be causing the issue.