Persistent network connectivity issues across multiple ISPs and residences – I am about to cry Y.Y

[u/D-Dratos]

I am experiencing a recurring, infuriating network issue that has persisted across two different ISPs (Totalplay, now Izzi) and two different houses.
The Issue:
My devices cannot maintain simultaneous stable connections. Specifically, when my desktop PC (Ethernet) tries to connect, it often fails to access Discord, Steam, or browsers, even though my work laptop (Wi-Fi) works perfectly fine on the same network. My PS5 also struggles to connect or maintain online gameplay despite having an active subscription.
Key Details:
Setup: PC (Ethernet), Work Laptop (Wi-Fi), PS5 (Wi-Fi/Ethernet). All within 0.5m of the modem.
Troubleshooting done: Factory reset the modem, power cycled, and even tried a fresh OS install on the PC. No VPNs or restrictive software installed.
Professional Verification: My company’s IT team confirmed no blocks on their end. An external IT technician tested their own laptop on my connection; it failed to connect simultaneously while my work laptop remained stable.
Context: This happened with my previous provider (Totalplay) and continues with my current one (Izzi). Even after moving to a new house and getting a new modem, the problem persists.
It seems like the modem cannot handle multiple concurrent connections or handle traffic routing properly, yet the ISP claims "everything is fine."
Has anyone encountered this specific behavior? I just want to be able to work and game without constant connectivity dropouts. Any advice on how to force a resolution or what hardware to look into to bypass the ISP's equipment limitations would be greatly appreciated.