this instruction is for comcast users, you may have a look: https://www.tp-link.com/us/faq-881.html not so sure whether it is the same for specturm

try to change 5G channel and check firmware update

How about wired speed of the Archer C9? if wired speed is normal, changing wifi channel and channel width usually improves the wireless speed, if not, maybe you can check the firmware update

you need to login tp-link web ui and check the internet ip there. if it is local ip, you need to open port for it on the modem. Then use the internet/wan ip of your modem to access the hard drive.

i don't know how to change it. you may contact tp-link support for help.

no, one APP can only setup a time zone for all the device. you can install the second one on another phone.