I like this idea and want to try this with my learning.
Is this what you are refereeing to?
https://www.amazon.com/Modbus-Chann...1681216126&sprefix=modbus+rtu,aps,101&sr=8-18
They are all pretty much the same thing.
I think I got this one
https://www.amazon.com/gp/product/B07ZLM3WL9/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1
But as long as it's Modbus RTU and you see it has the A,B terminals for the modbus connection, then it should be good.
Best way to start is to pull up a terminal and send commands directly before trying to go through a plc so you know it works.
Realterm has a decent interface for doing this.