will it works when directly connected to the modem/internet?

can you show us screenshot of that status unknown? also the log. Also check if there is any firewall or filtering on the router, disable it. the extender can do a firmware upgrade first.

do you put 300 in the min or max? it should be min then receiver will get 300kbps at least. Also make sure you have input the correct download and upload speed in the control settings. find more...

the firmware has no limitation for third-party firmware but those third-party firmware company need to update the firmware.

device time is not correct? you can change it under kasa -settings on left--location

nope