Another thought... Once PC-A has gone asleep, does a power down and up cycle wake it up? Then are you able to connect? If so, you could put a timer on the PC-A so it will power cycle at a set time...

Good that you are working now. Not sure either. Power down and up both the modem and router may help these unexplained things. I always power down and up the router after I have made changes so I do...

In most cases either would work fine unless you have some unusual layout and conditions. Buy from a company with a good return policy and try one out. If it does not work for you, return it.

The default userid and password is most times printed on the device. If not check the user doc. Most times it is admin/admin or such. If you changed and lost the user id/password, then you must do a...

Try using your actual ISP IPv4 address with the port number. Also try using the Tether app.

If it is working fine, I would just ignore it. If you have any issues or it just bugs you, try a factory reset and re-enter all of your settings.