You must be in the US on a cable based ISP with a regular modem (or a combo modem/router in modem/bridge mode) for this to work...
This has nothing to do with the Netduma itself. The fact that removing the Netduma from your setup gets you unbanned is because removing the Netduma changes the associated MAC address of your modem. In the US most modem's get dynamic IP addresses based on whatever device they are connected to. When the modem restarts with a new device connected to it, they see a different MAC address which gives the modem a new IP thus the ban is gone.
Now the bigger question is why were you banned in the first place? 99% of the time this is because of an infected or malicious device on your network. The hacked devices are part of a botnet and when it's master sends the command your device and thousands other devices start to sending a lot of data to the one specific destination and the destination service is unavailable because of that. It's called DDOS attack.
A few years ago it was Sony's PlayStation Network service which was under this kind of attack.This is just one attack, but there are tons of them every day and companies need to protect themselves. Sony decided to protect itself by banning IP addresses of known attackers.
You can fix this and continue using the Netduma by going into Settings > WAN
Enable Clone MAC and then change the last 3 sets of numbers/letters to something else. It can be 0-9 and A-F
Reboot your Netduma from misc settings. Wait for it to come back on.
Then reboot your modem and it should be back online.
If it doesn't work you did something wrong. Try again.