I find www.udemy.com is a https website. TL-WR841N doesn't support https website filtering. It will not work normally when controlling https website.

Not understand your language. Can you write in English?

How about the wired device？ Will they drop from the modem as well? It seems that you can still login to the modem's 192.168.1.1 page when connection drops. Did you ever contact your internet provide...

TD-W8901G and TD-W8960 use different chipset from different manufacturer. The bridge function of them are not compatible with each other.

Look here: http://whatismyipaddress.com/private-ip

Try to change the DSL modulation type under DSL settings to ADSL2+ and make sure the firmware is already the latest one. If the issue still exists, contact tp-link support for help.