Hello Siyab;

Modbus RTU drivers (free, integrated) have been made avaiulble for S7-300 CPUs (with a CP 341) only in TIA Portal.

To setup a S7-300/CP 341 as a Modbus master with Step 7 v5.5, you will need to purchase and install the Modbus master drivers shown below:

https://support.industry.siemens.com/cs/ca/en/view/27774018

And 

SIMATIC S7-300/S7-400 Loadable Driver for Point-to-Point CPs: MODBUS protocol, RTU format, S7 is master

https://support.industry.siemens.com/cs/ca/en/view/1220184

Hope this helps,

Daniel Chartier