On Tue, 27 Mar 2018 01:33:04 +0100, Adrian Caspersz
said:

You need a modern TV with a HDMI input.

Folks throw away such TVs these days for silly reasons.

Try asking on Freecycle (or your local equivalent).

I was afraid of needing a new TV but we have no plans to get a "modern TV".

I just realized though that it didn't even occur to me to look to see if
the PS3 had HDMI!

I just looked, and the PS3 does have an unused HDMI port!
http://www.bild.me/bild.php?file=6417873ps3.jpg

So can the Roku be set up in this configuration which exists now, all by
Ethernet cable?

WISP modem to Router to PS3 to TV where the PS3 has an unused HDMI port?