Ubuntu will use both your Wireless and your Ethernet at the same time. I use a program called
vnstat for Internet traffic monitoring:
rx / tx / total / estimated
Apr '18 20.83 GiB / 1.55 GiB / 22.38 GiB
May '18 4.33 GiB / 5.34 MiB / 4.34 GiB / 11.36 GiB
yesterday 404.97 MiB / 509 KiB / 405.47 MiB
today 593.05 MiB / 773 KiB / 593.80 MiB / --
Apr '18 206.79 GiB / 56.90 GiB / 263.69 GiB
May '18 63.19 GiB / 13.34 GiB / 76.53 GiB / 200.60 GiB
yesterday 6.73 GiB / 817.44 MiB / 7.53 GiB
today 4.03 GiB / 376.32 MiB / 4.39 GiB / 5.31 GiB
Today the WiFi (wlp60s0) has sent/received 593 MB and Ethernet (enp59s0) has sent/received 4.39 GB.
vnstat see this Q&A: How to track the total network data in a month
You don't have to tell Ubuntu to use Ethernet over Wifi because it automatically uses the fastest connection. The exception being if your Ethernet is 100 Mbps and your WiFi is faster than 300 Mbps. In this case the WiFi would take precedence.
If your WiFi is faster than your Ethernet click the Up/Down arrow representing network connection in the Systray. From the drop down menu disable the WiFi link by clicking
disconnect button. You can also disable the Ethernet link from the same drop down menu.