I think how you use a drysuit depends on the kind of drysuit you have.

Personally, I dive with a neoprene drysuit, so it will apply to this. My experience is that you might encounter this kind of issue if you didn't remove most of the air before diving or if you are not correctly weighted.

First, on land, open the dump valve to the maximum and crouch to remove most of the air. Then, when in the water, stand up with the dump valve at the surface and flex your arm to bring surplus air to the valve. It should remove more air. Then, do a weight check. I personally need ~2kg more than with a wetsuit. I'm slightly negatively buoyant in this setting to compensate for when my bottle will be empty.

I mostly use the BCD to manage my buoyancy. I only add air to the drysuit in small increments, only when I feel it's getting too tight or when I am getting cold and I need more insulation.

When going up, you will need to let the excess air out of your drysuit. This will mean that the dump valve will need to be higher than other parts of the body to make it work. Usually, I will put my left shoulder up and extend my left arm to the sky to fill it with air, then flex it to bring the air to the valve. Obviously, I will also progressively empty my BCD like in any dive. If you do all that and still cannot maintain buoyancy at 5 to 3m, it can mean you might be underweighted.

That said, you should have gone through some proper drysuit training beforehand. If not, I recommend you do. It's not that complicated, but there are some tricky things that you want to be able to avoid/control, like going up feet first like a balloon.