The solution is to contact the 20i support and ask them to whitelist the IP address or UserAgent of the MainWP Dashboard.
The User-Agent that MainWP uses is:
Mozilla/5.0 (compatible; MainWP/4.0; +http://mainwp.com)
The IP of the Dashboard can be found on the Dashboard > Info > Server page, in the Server IP field.
The server IP field may sometimes show a local address (such as 127.0.0.x, or 10.x.x.x). In that case, you can look for the actual IP in the host’s Dashboard, or you may contact the support of the host where the Dashboard is located and ask them for the correct IP address.