What may be causing my Wi-Fi connectivity to degrade throughout the day? (FritzBox 7490, macOS 14.7.1)

I have been facing the following issue for at least six months, gone through it with my ISP’s support but since they appear not to have a clue, I am hoping to find at least some understanding here. I have been having a FTTH 1 Gbps service with my ISP for the last 3.5 years. In general it has been quite good, with Wi-Fi speeds reaching up to 750/350 Mbps across my devices (all Apple) and across the apartment (50 m2). Around February 2024 download speed dropped to 20 Mbps: we replaced the modem/router (a Huawei all-in-one machine). Around May I started to experience a strange issue: everything w

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