Do you set it work at Access Point mode or Range Extender mode? If it's latter, we can't change the channel because it's should be the same as your main wifi.

Do you mean the TL-WA850RE using too much connection and slow down your speed on the router?How many devices are connecting to TL-WA850RE? How about trying to set bandwidth control on your router?

The extended Wi-Fi password on TL-WA855RE should be the same as your main Wi-Fi. And we can't change that.

You mean other uncontrolled devices still enjoy the normal speed with thoes same websites? If you disable parental control, do the same devices get back to normal? Do you set any other bandwitdth...

Did you try to login with wired connection? and you can also check the IPV4 default gateway on the pc which is connecting to the router. Use that numbers to login or try with another computers.

is there any ip address with wireless connection? have you changed another wireless channel?