As mentioned above, bearing savers are an interference fit.
First of all - reassemble the bits you've taken apart
You don't need to take them apart to get them off, or grease the bearings. The spring is to push the black disc in towards the hub (with the grease behind it). You'll see a grease nipple in the middle of the black disc - that's the bit you need to ensure you keep clean and use regularly
Once you start pumping grease in to them, you'll know when they're full as grease will squirt out of the pin-hole overflow hole on the side of the cylinder.
I'd be very surprised if Indespension had supplied a trailer with poorly greased drums. It may have happened, I think it unlikely, but it's obviously worth checking.
Jack the trailer up. Take the wheel off. Use a soft face mallet (or, as mentioned, put a block of wood between a ball pein hammer and the bearing saver) and hit the chromed, side face of the cylinder. Turn the wheel 90 degrees and do the same. Do this, rotating the wheel, you won't need to hit it particularly hard, and you'll soon see it start to move.
Eventually it'll come off and you'll see the hub nut, with split pin through it, underneath. Look inside the bearing saver and you should see plenty of grease - this is the blue stuff. In my experience this blue grease is a perfectly good, lithium based, hub grease. I use it myself on services. Bearing savers are actually one of the bits of trailer kit that do live up to the hype! I've had a set on my Roller Coaster 1.3 for donkeys' years and my last set of bearings lasted eight years, with minimal service work (it's a bit of a busman's holiday
If the black disc is near the outer face of the bearing saver then it should be full of grease. If it's sunk in, towards the drum, you know it needs filling up with grease.
If that's clear as mud, feel free to drop me a line, I'll happily talk you through it.