I have just installed a brand new RX5500XT on my PC, but since then the right refresh rate for my monitor is not available anymore, and any other refresh rate i choose causes poor quality graphics. My monitor is a Flatron M2280A and works with a refresh rate of 59.934Hz, but the only refresh rates available are 50,000Hz, 59,940Hz, and 60,000Hz. I use a fully updated Windows 10, and all drivers are up to date too.
Processor: Core i3 6th gen
Motherboard: Asrock h110m-itx
Monitor: LG Flatron M2280A
anything between 59 and 60hz should work with your Resolution of Full HD 1080P.
The 59.940hz should work fine with your 1080p Monitor.
I would check your LG Monitor's OSD Menu to make sure you have it set for PC and the type of connection you have from your GPU card.
Make sure you are using the display port cable, the hdmi may limit the refresh rate selection.