What went wrong?

[FDS]

Buy a dedicated BEC (Battery Eliminator Circuit) to get a proper stable 5v, they are cheaper than buying new receivers and will allow you to run with the battery you have. Ideal for testing, reliable and then you can put it in the plane.
If you don’t want to spend as much on a new receiver then the Lemon RX ones are really good, light and great reception.

 

[moret]

oops..
- Can I use two aa batteries as a replacement for a Li-Po?

A standard new AA battery is around 1.5 volts. Two would only give you 3 volts so 3 would give you 4.5.
If at all possible just get a ESC with a BEC or a stand alone BEC. If you are going to run a brushless motor on this project, you will need a ESC anyway

 

[Wildthing]

Buy a dedicated BEC (Battery Eliminator Circuit) to get a proper stable 5v, they are cheaper than buying new receivers and will allow you to run with the battery you have. Ideal for testing, reliable and then you can put it in the plane.
If you don’t want to spend as much on a new receiver then the Lemon RX ones are really good, light and great reception.

That's why I use an old esc, doesn't matter if I have a 2S, 3S or even 4S battery laying close by, just plug it in and away I go.

 

[FDS]

I have a crappy 20A one that always gets hot when you run a motor on it, it’s my power supply for binding!

 

[Wildthing]

I have a crappy 20A one that always gets hot when you run a motor on it, it’s my power supply for binding!

LOL, 20A also , mine gets warm with no motor on it and actually won't power up a motor anymore and instead of throwing it out I said it is perfect for bench testing.