Equill has likely hit the nail on the head there.
Failing this, however, there is also the question of what software you have on the machine. What System? Are you trying to change resolution in the relevant Radius Control Panel (I think it's Dynamic Desktop, but I am not in a position to check this right now), or in the Monitors and Sounds Control Panel (bad choice)?
The devil is in the detail.
In my experience, the ColorSync software in the default installations of OS7.6 - 8.1 does not always play particularly well with some large monitors, even Apple's own brand of monitors, so that a little fiddling is sometimes needed. E.g., you can occasionally get somewhere further along with the Monitors Control Panel installed typically in the Apple Extras folder. Why the fiddle I can't say. You could also try disabling Monitors and Sounds in your Extensions Manager and then play with the Radius software without interference from the default System installation.
A Radius card ought to work with a Radius Monitor, though the PrecisionColor 24XT is not going to do millions of colours on a 19" colour CRT, and it well and truly pre-dates release of MacOS 8.1, so there may be compatibility issues if that's what you are using. My 24XT does, however, work very nicely with my two page fixed resolution Radius 20" greyscale in my 840av, under a variety of OS setups, including MacOS8.1. So it stands to reason that it should drive a large resolution of some description on your 19" Radius in your 840av.
However, almost no Nubus card will give you better video output than the 840av does natively, and perhaps none will, since Nubus is relatively slow (though those in the 840av are double speed). If your monitor is working from onboard video, you can confidently remove the 24XT as an unnecessary encumbrance, in order to get yourself a nice little IIci and stick it in there. A 24XT is not a high end card for an 840av.
I would also be surprised if a Radius 19" monitor offered a resolution higher than the one you have already gotten, though an 840av will indeed go higher than 1024x768 and will, as I recall, do millions at 1024x768 if it has the vram upgrade of 4x256k vram simms.