I am far from an expert on the Serial binding. But I do know it is one of the more challenging ones to get working correctly. Is it possible the baud rate isn’t set right which is causing the Strings to be cut into parts? I really don’t know if that even makes sense. It might be easier to make it work with an external script to push the data to OH if messing with the settings for the binding can’t help.
I’d start with How to get started (there is no step-by-step tutorial) because this question belies a lack of understanding of how OH works over all and the scope of the answer is far to great to quickly type out here.