Fist you should update the motherboard BIOS to ensure compatibility with the planned CPU:
https://www.msi.com/Motherboard/support/H110M-PRO-VH
Mixing RAM isn't a good idea, doing so can cause plenty of stability issues, but if you must, try to get an identical stick to the one installed, if that's not possible, remove it and inspect the ID label, it'll detail the speeds and voltages the module runs at, you should match any new module to those specs.
With memory so expensive ATM, you may also want to consider looking through E-bay and Craiglist ( or local equivalent ) for a used matched pair of modules.
Don't fret if you can't find modules of 2133MHz speed, the MB will run faster memory, but at its own 2133MHz speed limit.