"arp -a" proved useful only in that it did not list any other network computers/devices.
I expected to see at least two or three devices having 192.168.1.x IP addresses. Does your network have any devices assigned a Static IP address?
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By "doing the same", I meant having other network devices being connected both wired and wireless at the same time.
Ensure that all network devices are either wired or wirelessly connected. One or the other - not both at the same time.
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You should be able to log into the router's Admin pages from any network computer be it wired or wireless. And the wired connection does not need to be direct PC LAN port to Router LAN port.
Also, just for the record, when the router was originally configured you did change the default user name and password - correct? Not asking for them and do not post them in any case.
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Take a look at your network devices as the router is seeing them.
Per Page 121 of the User Manual - take a look at the Attached Devices list.
Do a screen capture (or captures) as necessary to show all connected devices and post via imgur (
www.imgur.com).
I think there are network problems but those problems would not be website specific, as per your original post.
One problem masking another....