At least one motor is worn out. Brushed motors in quads don't last long. The flight controller is doing it's thing keeping the quad flying and compensating for the weak motor.
Read the manual that comes with the receiver. The default on some is hold position of controls. Which is fine for if your model goes behind something traveling sideways to your view. If it's flying away from you when it loses signal then it just keeps going and flies away and out of your life.
It's important to set up the fail safe on the receiver for what you want it to do if signal is lost. I set mine to level out and about 1/4 throttle so it won't drop like a rock. And then test it before you fly it.
Last year after seeing a friend fly hex and a DJI Phantom i searched for drone on ebay. And did some reading and watching youtube videos. I bought a Proto X SLT. It flew well but was to small for outdoors. Soon after I had a Syma X5C. I really learned to fly on that one and in a couple...
I live at 7000 and have flown quads at up to 10,000 ft. I've never flown them at lower elevations so I don't know what I'm missing. I notice a little less thrust at 10k.
I recently bought a 250 racer RTF and suspect it was setup at sea level. It came with 5030 x 3 props, I replaced them...