How to isolate server traffic using VLANs with limited router capabilities?

I am a beginner in networking and I currently have the following network setup: Main Router: Huawei Optixstar with only 1 VLAN for internet access (no option to create new VLANs). It handles lan and wifi access. DSL Router: Xiaomi configured as a bridge to manage LAN and Wi-Fi. It is connected to main router via łan. Active Switch: TP-Link TL-SG105E (Layer 2 switching). Connected to main router via lan. 2x Wi-Fi Repeaters: Xiaomi devices. Connected to main router via wi-fi. End Devices: Various devices connected via Wi-Fi to main router, access points or DSL router. Problem Description: I have

Question by newman on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.