@DollaBillz 

Hi,

In the 5 GHz band the phone will connect to the AX1500 in 802.11ac mode with probably 80 MHz channel width.

However, the 802.11ac standard has never been specified for the 2.4 GHz band and also no 80 MHz channel width. Hence, in the 2.4 GHz band your phone will connect using 802.11n and in best case with 40 MHz channel width.

That all makes a big difference.

Furthermore, if you have left the wireless setting for "Channel Width" at it's default "Auto" on the AX1500, then you might actually be running at only 20 MHz.

So you could try if fixing the "Channel Width" setting to "40 MHz" results in a somewhat higher data throughput.

But it's also possible that your phone just doesn't support a higher speed in the 2.4 GHz band than that what you already got.

@woozle I would fine that extremely hard to believe. I've tried it on my Motorola phone, my brand new iPhone, my 4 month old surface go, multiple fire hd tablets, etc, and none of them will get a speed above 30-50Mbps when on the 2.4Ghz band, but will get the full 200Mbps on the 5Ghz band.

@DollaBillz 

Following this link you can find the specification for the Wi-Fi speeds of the AX1500:

https://www.tp-link.com/us/home-networking/wifi-router/archer-ax1500/#specifications

The theoretical maximum for the 2.4 GHz radio is 300 Mbps. (this is if both 2.4 GHz antennas are being used)

However, practically achievable real world speeds are usually just 50-70% of the theoretical speed, and that is if router and client device are located within the same room (no walls in between).

So, when connected to the AX1500's 2.4 GHz Wi-Fi, client devices with two antennas should be able to attain data transfer speeds of about 150-200 Mbps and client devices with only one antenna around 75-100 Mbps.

How many Wi-Fi antennas a phone or tablet has is often not easy to determine. According to information I found on the Internet the iPhone XR/XS and all newer iPhone models are said to have two.

But since you are already maxing out at 50 Mbps something else must be happening.

Another important factor for the attainable speeds is Wi-Fi congestion. Especially in apartment buildings the 2.4 GHz band can be very crowded, as nowadays almost every household has a Wi-Fi router and several devices that use the Wi-Fi.
Depending on how severe the congestion is, the attainable Wi-Fi speed could drop a lot.

Besides Wi-Fi there are also other possible sources of interference that can reduce Wi-Fi speeds in some cases. Like Bluetooth or other wireless products that work in the 2.4 GHz band, even analog wireless CCTV.

All this can make troubleshooting of slow Wi-Fi speeds rather difficult.

Do you perhaps have another previously used router at hand that you could use to countercheck the 2.4 GHz results of your AX1500 at the same location?