Dragontouch M7 tablet doesn't turn on after replacing the battery

Hi, My DragonTouch M7 tablet stopped lighting up (the last sign of life was a flickering of the screen). The battery started to inflate. To replace it, I followed a tutorial about a model from the same manufacturer.

The wires didn't pass through holes in the board, they were simply soldered on the surface of the board.

The desoldering of the old wires was a little more difficult than expected because at the beginning I had set the iron to 200°C and I made the mistake of putting a too fine tip (wanting to be precise). Even at 400°C, I could touch the solder with the iron for several seconds, but it didn't melt (I even twisted the tip, which I was less able to straighten afterwards)

I understood afterwards and I used a bigger tip. The change of wires was very fast, I didn't add any solder, I just removed the old wires and soldered the new ones.

Unfortunately, the tablet doesn't light up. What's the best way to check that there is no bad or poor contact?

I've attached a photo with the two solder joints in question, and I'll have a multi-meter available this weekend. Maybe there is a mistake I made during the welding process.thank you (French or English) or for any help in understanding what is going on.

(Disclaimer: I posted this question in english and in french. If it got solved in one language, I'll translate - or let the answerer to do so - in the other one.)