How to use 2 FullHD monitors to one Type-C port on Macbook Pro 15 2018?
The Startech adapter you've got works as a MST (Multi-Stream Transport) hub. MST is a part of the DisplayPort 1.2 standard, which makes it possible for monitors and computers to support "daisy-chaining".

Daisy-chaining is most often seen where the monitor has both an input and output DisplayPort adapter, allowing you to connect the first monitor to the computer, and the second monitor to the first monitor. If both monitor and computer support MST daisy-chaining the two displays will act as seperate displays, exactly like if they had been plugged in directly to the computer. Note however that the monitors share bandwidth, so you wouldn't be able to have them at the same resolutions and refresh rates as possible when using seperate cables connected to the computer.

The problem for you is that macOS does not support MST daisy-chaining. I believe this is a software limitation and not a hardware limitation. Instead macOS supports daisy-chaining Thunderbolt monitors (such as with the Apple Thunderbolt Monitor).

Therefore it is not possible to get your StarTech adapter working with macOS. Indeed StarTech also lists that it is only compatible with (some) Windows PCs.

However it is certainly possible to connect two Full-HD displays to a single of the Type-C ports on the 2018 MacBook Pro. These ports are really Thunderbolt 3 ports, so you have a lot of options here. You can buy both adapters as well as full-fledged Thundebolt 3 docks that allows connecting multiple displays over DisplayPort/HDMI as well as adding extra USB-A, USB-C, Firewire, Ethernet, Audio, eSATA or other connections as well as SD-card readers, etc. Some of these even supply power to the MacBook Pro so that you can charge your MacBook Pro over the same, single cable.

An example of such a product is the OWC Thunderbolt 3 14-port dock:

https://www.owcdigital.com/products/thunderbolt-3-dock-14-port

They also have other variants such as a 12-ports dock and a dock with FireWire 800.

You can also get similar products from Belkin, Elgato, Caldigit, Iogear, etc. Even StarTech produces one (same make as your adapter):

https://www.amazon.com/StarTech-com-TB3DKM2DP-Thunderbolt-Dock-DisplayPort/dp/B07BJJX47G/ref=as_li_ss_tl?ie=UTF8&amp;linkCode=sl1&amp;tag=reality&amp;linkId=b1418a14da750a87c5d7c2a099401212

That is a $123.99 dock/adapter with 2 DisplayPort, 1 USB-A and 1 ethernet connectors.