If the RSSI is less than 4-5, typically, the device won't connect. The network transmits a minimum signal strength below which devices cannot connect. This value is typically -104 dBm, which translates to a RSSI of 4-5.
Regarding the measurement of -75 dBm, if you're not able to connect to this network, it's likely a network for which you do not have access.
To ensure you don't have an issue with your device, put the test SIM in a reference device and verify the network coverage; i.e. you can connect with the reference device.
(i see this is an old post , but I may still help you, or others on the dev community)
Probably your device is a TC65.
In order to understand better what is happening I would suggest you to provide the result of the following commands:
Check if the radio band is correctly configured to the appropriate carrier band.
Finally, it's also important to understand which antenna you are using and where it is positioned. If you device is metalic and the antenna is internal, you might really not get signal...