Hmm that's weird. It used to say it's 75hz on lg's website, idk why they removed it. But on newegg is says it's 75hz: https://www.newegg.com/amp/lg-25um58-25-ultrawide-full-hd/p/N82E16824025340
You can also check any article about this monitor and it's very likely that they will mention it's 75hz.
And also I do know how to setup custom resolution in nvidia's control panel and I already did it, but the monitor thinks it is receiving 16:9 image instead of 21:9. BUT when I go to monitor's OSD and set the image to 'strech' instead of 'original' everything works like it should. And the problem is I can't have it like that because I play some games on 1920x1080 and I don't want those games to be stretched.
Edit: My GPU is gtx 1060 3gb. My cable is a no-name hdmi, but I've tried it with different cables and it doesn't matter, still the same thing.