You have a KR808D-1, same threading as Vapor4Life, SMokelessImage, Bloogplanet, Halocigs G6 and others.
Best way to refill may be to throw out the carto (it is not what we call a cart - it's a cartomizer aka carto) and instead buy ones that are designed to be refilled like the Bloogplanet ones. (same threading)
Some cartos just don't refill well. Some rebranders sell cartos with glued on end caps or hard endcaps that are hard to remove without damaging them. Some use cartos with poorly structured insides that basically work well for one fill.
You can try to pop out the endcap (stick an unbent paper clip only 2/3 to 3/4 of the way into the mouthhole and poke it against the inner wall of the endcap and see if you can pry it out. If it comes out, you can add juice. However if there is an inner silicone seal (shaped like a flat donut) you would need to remove that to add liquid and in my experience those style cartos may suck when reused.
I recommend thin juices, 20% or less VG though 30% may work OK. I also recommend NOT fully using the cartomizer because you can singe them, leaving a bitter flavor. Just keep the carto damp with liquid.
You probably have automatic batteries so you also have to be extremely careful that the carto is not going to leak before you put it back on the battery.
--- this is how I fill good 1ml capacity (normal size) carto blanks with a defined center air channel
When I fill the first time (and whenever adding a lot of juice) I hold tissue against the threaded end hole of the carto. Tilt and drip in 18 drops slowly (wait after the first few which may take longer to soak in). Then I wait 15-20 seconds. Then I see how many more drops the stuffing will soak in easily. Then I do cleanup of the threads, then tilt open end down into a tissue and puff into the threaded end hole to make sure the air channel is clear, then while still pointed down I touch soft tissue to the top of the stuffing to sop up any excess (helps prevent leaks). Then to settle it I let it lay on different sides for 15-20 minutes to spread the juice to every area of the carto. 15-20 minutes is more than enough settling with the thin juices I use. Only the first fill needs some settling time (to make sure every bit of stuffing is damp).
As I vape, I keep it damp. If the vapor seems light or dry or tastes even a tiny bit off or if the carto is getting abnormally warm, I see if it could use some juice. And you can also check the top of the stuffing - if it has gone white you could probably add a bit of juice before heading out or putting a new battery on it etc.
I use thin juices, PG w/ 20% or less VG and a couple of Vapor4Life WOW juices. If I get a juice that drops a lot of flavoring sediment, I find a different juice.