Teclast is an android tablet. I've never handled one before, but can suggest the following:

Try to connect other devices on that wifi network and see what happens. If they can connect, then the problem is in the Teclast.

Check the tablet to see if all wifi connectivity permissions are allowed

Check any firewalls you have on the tablet to see if net access is being blocked

Back in the time of Android Honeycomb, I had a friend who used a Teclast 8" tablet mainly to play Plants and Zombies. One day she grumbled that she had to do a factory reset cos wifi wasn't working. It started working after that reset. At that time I was considering to buy a tablet for myself, hence my interest in the matter. I ended up with a Samsung Galaxy 5+ phablet instead (phone in a 7" tablet).

If a full reset doesn't work, then it may be a hardware problem. Wifi transmitter/receiver components are like little radios which can and do wear out over time. One last possibility: maybe the antenna became disconnected after a fall?