Hello, welcome to the community. 🙂
If you don’t have any big problems with ethernet cables, then everything is fine from the UPC side.
WLAN is a wireless signal, so the signal and strength can vary. There are many factors that play an important role in WiFi that you cannot always affect.
But from experience I can tell you that an additional router can help to solve your problematic. You can then set our modem to bridge mode, but we must first set your connection to IPv4, otherwise you cannot activate bridge.
As soon as you have bought the external router, you are welcome to contact us again so that we can activate IPv4. I like the Huawei router you sent. What I can also recommend is Netgear Router.
Best regards, Cem