Well, if I were you, the first thing I'd do is unplug the ethernet cable from your iMac. Then, switch in wi-fi via the network preference pane. Try to connect your iMac to the internet via the modem's wi-fi signal. You'll have to enter a user name and password. These should be on the back of the modem. Hit return. If you can access the internet via a browser on your iMac, you're half way home.

You can check your download speed via an online checker, like Speedtest.net

If you're getting around 10 Mbps or better, buy a HDMI stick for your television, Chromecast, Amazon Fire Stick, Roku... you will be able to log on to your modem using the same password as you did for your iMac.

If it won't get a signal or buffers too much to be watchable, you'll have to consider an wi-fi signal extender: powerline or mesh. Lots of reading to do!

Good luck.

iMac (19,1, 3.1 GHz i5, 12.7.4, 40 Gb RAM); MacBook Air (1.8 Ghz, 8 Gb RAM, 10.14.6, 256 Gb SSD) Vodafone router and Devolo Wi-Fi Extender, Canon TS8351 printer/scanner.