Searched the forums for a similar issue, and couldn't find the same scenario.
First, the hardware:
- Motorola Surfboard SB6121 modem
- TP-Link Archer C9 Router
- Window 10 PC
- Various connected WiFi devices: 2 cell phones, 3 alexa's/dots, Sony Smart TV, Laptop, WiFi printer, Chromecast, PS4...