User manual: Stront40

TL;DR

Use a USB-A to Type-C or Type-C to Type-C cable to connect your keyboard to your PC.
Connect to the right side of your split keyboard. Avoid connecting to the left side to prevent unstable cursor movement.
Never unplug or plug-in the Type C or TRS cable while the USB-C connection is still active. Doing so can cause issues. Some people experience problems even after making this mistake once, so be cautious.
The type-c port in the middle is used to connect between splits only.
Insert your switches directly to your board. MX build comes with poron foam. For Choc builds, there’s a gap between the switch and the switch plate which is normal, this allows you to use a switch puller.

First, connect the left and right halves of your keyboard using the TRS/TRRS cable. Ensure that the cable is fully inserted but not forced violently.
Insert a USB-C (or Type-C) cable into the right side of your split keyboard.
Note that you don’t need to insert a USB-C cable into both sides; only the right side requires it.
Demo video.

Use Vial to remap the keys. You can download the software or use web interface to remap the keys. You may refer to Basic Vial Usage and Keymap Idea to explore Vial and keymap.
Auto-detected in Vial, supports Win, Linux, and Mac. Real-time key remap software without the needs of code compilation. No load json is required.

Check any bent pins on your switch.

Launch one application each time. Use Vial instead of Via. Do not launch together to avoid conflict.

Source code from zzeneg/40keys.