We will need that you contact your sales manager representative and provide him (only him) the UserName of your RMS account so he can send a ticket to support.
Also you will need you provide him some Serial and MAC number of devices that are failing to connect.
Most of the times there are 3 things that prevent a device from connecting to the RMS
1. Device has no Internet connectivity.
2. RMS is turned off on the device. Or not enough licences in the RMS Pool.
3. Very Old firmware version in the device
I think no. 3 is most likely in this case.
Old versions are using using old OpenVPN protocol to connect to RMS, and actual versions are using MQTT. The old version is not available anymore so I would advice to update the firmware
By providing that information we can check iff there is any issue regarding your case.