Note: I’ve the DJI Standard 3.

So I’ve downloaded litchi and I set my way points around an area of (400/600m) distance and maybe (100/200m) Of height. I lost signal around the 7/8 waypoints out of (12). The drone still recorded the whole way and then arrived back to me.

My questions:
1) Why did it do that? - Can I set waypoints beyond the drones limits and it’ll come back to me?

2) Is there any accessories to buy to extend the range the drone can go?

Note: I maximum distance I reached with this drone was (Height: 400m) & (Distance: 535m).

* Could have reached further maybe but was in a neighborhood signal was weakened

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RedHotPoker

You could try a set of parabolic range extenders cheap and very easy to fit should do the job nicely for you.
Just make sure remote is pointed towards the drone at all times they are very directional.

When using Litchi, if you programme a waypoints mission and upload it to the drone, assuming you have ensured sufficient GPS satellites locked, once it sets off, it will follow the waypoint track even if the RC loses contact with the drone. That's what it is designed to do.  You should ensure that the battery is sufficiently charged to complete the planned flight. But if the intelligent flight batteries detect a critical point of charge, the RTH will initiate and bring the drone back home.  This is fine if there are no obstacles higher than RTH altitude between you and the trigger point when this occurs.   Planning and re-checking are essential if you're going to attempt to do this. Bearing in mind that you'll be outside the guidelines of the drone code ( ie keeping the drone in VLOS all the time.)