Do not worry about the warning.  That happens on my modem. Just a certificate issue. Proceed and make the update. 

Thanks! Say, another weird thing is that on one laptop, the first to use the ARRIS and near the modem, the username (the 2.4G) is G34-66CD (and only that one for the ARRIS), and the second laptop to connect, which is on a different floor, has only the username G34-66CD-5G...and the download speed is more than twice as fast. But I can’t get G34-66CD-5G, the correct one, on my first computer! I don’t get why it’s happening like that. Any ideas and/or solutions? Again, thank you so very, very much! Whether or not I hear from you again, thank you so very, very much! Take care, and you and yours stay safe and healthy!    

Could you let me know the model of the computer on which you are unable to locate the 5G network?

SONY VAIO Device name    LAPTOP-159OU6I
Processor    Intel(R) Pentium(R) CPU B940 @ 2.00GHz   2.00 GHz
Installed RAM    8.00 GB
Device ID    F1C49136-C913-44E7-9CAD-E8007394998A
Product ID    00326-10000-00000-AA859
System type    64-bit operating system, x64-based processor
 

Since you are unable to connect to the 5G network on your laptop, I would suggest changing the security mode.

Login in to 192.168.0.1>Gateway>Connection>Wi-Fi>Networks>Edit>Security mode is WPAWPA2-PSK (TKIP/AES).

After changing the security mode, if the problem persists, I request you to contact laptop support for further assistance.

Problem solved using a TP-Link AC1300 USB adapter.

But still unable to change the user name or password! Changing the password is more important! 192.168.0.1 does not work.

To change the username and password, please follow the steps on the below given link:

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Changing-the-Wi-Fi-Name-and-Password