You need to do this while connected by Ethernet to your router. There are too many issues which can affect Wi-Fi to allow troubleshooting.
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Internet speed at the modem is for all devices not just the one device you are measuring from. Make sure you are aware of what activity is occuring for all devices on your network when you are doing your tests. This is on top of any WiFi congestion and channel switching issues you may be experiencing which is why @NFtoBC suggested you do your speed tests over Ethernet to eliminate those factors.