Connection problems can occur in case of errors in the configuration of the communication, about which there is little information provided in your post. I recommend reading these materials (if you haven't already) and double checking your configuration parameters and code. The first link relates to a different CM (and PLC), but basic requirements and code should be the same.

Master-Slave Communication via a CM PtP using the Modbus RTU Protocol

How do you establish a MODBUS-RTU communication with STEP 7 (TIA Portal) for the SIMATIC S7-1200?

Additionally, I found a few basic blogs an videos guides by googling "how to configure cb1241", so you could check those as well.

It also helps to have some sort of USB-RS485 converter and Modbus RTU software (pretty easy to find freeware) to test communication with the CM and Modbus device individually using a laptop - that way you're debugging problems one device at a time instead of wondering whether it's the device or the CM that has problems.