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.