Though I haven't bought this model, it's the fastest mobile device in TP-LINK. I used M7350 before which is also good. Since M7650 is better than that, I think it should be good to have a try:)

Are the powerline LED light still on? If they are off, we need to repair them. Certainly, make sure your main router is working properly. Firstly, refer to the link to leave out the powerline...

When you put static DNS on your modem, is your modem working properly? How about if you change the LAN ip address of TL-WA850RE into 192.168.1.X?

Have you reset the camera to connect to the new modem? Here is the link to guide you connect it to your home wifi network. https://www.tp-link.com/au/faq-820.html

The time should be synchronous with NTP server automatically? Also can't found where to set it manually.

oh, English better:-) Do you mean you change the antenna but there is no improvement? We'd better change all the antenna if we want to change that. Otherwise, there is no difference.