Kindly heck and ensure that your installed RT-Lab version is compatible with your MATLAB version. Compatibility issues might arise if the RT-Lab version you have installed does not support the MATLAB version you're using.
Then you can manually add the RT-Lab library in your Simulink environment just by running a script within MATLAB.
The script is located within your RT-Lab installation folder: C:\OPAL-RT\RT-LAB\[RT-Lab version]\simulink\m\setup_rtlab.m
Also check the Installation guide: https://www.opal-rt.com/wp-content/uploads/2020/09/RT_LAB_Installation_Guide_v2020.2.pdf
