Roy - DSO will require different settings than planets. Planets are very bright and only need exposures that are fractions of a second. DSO will generally require several seconds. With a 4 inch aperture at f/7, I can't get very deep (or clean) with a single exposure. With stacking, I have been able to capture DSO as dim as magnitude 15.5 using 4 second sub-exposures that were stacked for 10-15 minutes of integrated exposure.
Your camera has an IMX294 sensor and is essentially equivalent to the ZWO ASI294. People have achieved excellent results with that camera. It has a large sensor, and when paired with a fast Newtonian, will provide a field of view that is big enough for all but the largest DSO. This is important because stacking tools need to capture surrounding stars to align the frames (they generally need three, but can use more if present). Not sure if I got your scope right in the image below. The numbers I found in one of the tools I use indicated it might be ~f/4.
You didn't give any details of the settings you were using, but there are a few things you can try.
First, aim at a star you can see in a finder. I like Polaris because it doesn't move around much and it isn't super bright. There are a couple of reasons to do this. One, you know you have something in the frame, and two, you can make sure you are focused. Polaris is a double star and good focus should bring out both components. An exposure of 1 second is a good place to start. You may need to dial it up or down (probably down with your massive 18 inch aperture!). Also make sure the histogram is un-stretched (low, mid, high levels unmodified) and the image is not over saturated (curve not bunched up at the far right of the x-axis).
Next, try pointing at a globular cluster. They are relatively bright for DSO, and are good starter targets. An exposure of 4 seconds should be a good place to start. If that isn't enough, increase the exposure. You may find a couple of things limit how much exposure you can use. Alt-az mounts have a practical exposure limit of about 30 seconds, depending on what part of the sky you are pointed to. Going longer will streak the image from field rotation. The quality of tracking may impose an even shorter limit. Any jerky motion can streak the image. My alt-az setup has a hard time going past 4 seconds without experiencing some kind of bump or jiggle that ruins an exposure.
Watch the histogram as you change exposure. It will tell you if you are underexposed, or overexposed. Without getting into too much detail, you want the curve to have some gap between the far left of the x-axis (zero), and you don't want the curve bunched up at the far right of the x-axis (100%). If there is no gap between the curve and zero, you are underexposed. If it's bunched up at the far right, as previously mentioned, you are overexposed.
With an 18 inch aperture, you may see a lot more than I do in a single exposure, but for very dim objects, you will probably have to stack multiple subs. Single exposures can be noisy. Stacking subs cleans this up and reveals dimmer objects. Sometimes I can't even see the dimmest objects until I stack for a while. You may also have to enhance the stack by adjusting the low and mid levels of the histogram. This is done to take out sky glow, and brighten darker parts of the image. It's hard to describe the process in words, but it involves moving the low level to the point where the curve starts "uphill" and moving the mid level progressively closer to the "peak", but always on the downhill side of the curve.
In summary, start with something that you know is in the field of view. Get a feel for exposure times. And learn how to use the histogram. Starting with easier targets will pay off when you go for the dim, deep targets.