@Xave 

Can I know what is the device connecting to the 2.4G network, and what is the link speed between them? If it is a laptop, you can check the link speed as follows:

How to check the associated/link speed on a wireless client

I would suggest you read the following story shared by TomHunt, he gave very clear instructions to check if the speed is normal:

How to know if my clients get the normal speed?

@Xave 

Are they both connected to the 2.4G network? Can I have the screenshots of the link speed on both the laptop and phone?

Here you will find the difference between the theoretical wireless rate and the actual wireless rate, the actual wireless rate generally only reaches 75% of the negotiated rate between the client and the router, and may only reach 30% to 50% in an environment with large wireless interference. If the link speed on the laptop is only 65mbps, and there is heavy congestion, then I assume the actual speed may not be good. You can install a Wi-Fi analyzer to review the wireless networks around you, then try choosing a channel that is less congested with others, and change the channel width to 40MHz to monitor the connectivity again.