have you calibrated the SDR dongle?
best way if to use WSPR signals as they are very narrow band and on most bands
if you are using something like SDR# you will have 2 parameters for calibration one is the PPM to set the TCXO and an offset
to set the PPM its normally best best to use the highest frequency you can, then use something like one of the beacons (i use GB3UHF and GB3VHF) about 30 miles from me)
if you are looking at HF i have a page on my QSL.net site that maybe useful for you https://www.qsl.net/m0pwx/modes-and-freq.htm it has the most common digital mode frequencies and the software you need to decode them
(Not Pete M0PSX who runs the site)