Why is my Wi-Fi Router showing link speed of 72 Mbps?

The maximum Wi-Fi link speed (PHY rate) a given client can get when connecting to a given AP is the intersection of which speed-increasing factors both devices support (the overlap in the venn diagram of what they support).

Smartphones — even top-of-the-line models — often don't have the power and thermal budget and physical space required to support all of the speed-increasing factors that a top-of-the-line AP, laptop, or desktop Wi-Fi card can support. I've never seen a smartphone that supports 3 spatial streams or more, even though 3x3 APs and laptops started shipping in 2009.

So if one device supports 2 spatial streams but only 20MHz-wide channels, and the other device supports up to 80MHz-wide channels but only 1 spatial stream, the subset that they both support is 1 spatial stream and 20MHz-wide channels.

With 1 spatial stream and 20MHz wide channels and only 802.11n modulation schemes, the max PHY rate you can get is 72.2 Mbps. It seems likely that this is the situation you're in.

The only piece I'm unsure about is your OnePlus 3. It's hard to find reliable information about how many spatial streams it supports. It uses a Qualcomm Snapdragon 820 SoC, and at least one variant of the 820 supports two spatial streams. It could be that your OnePlus 3 uses a different variant that only supports a single spatial stream, or it could be that OnePlus decided to save battery and space by only enabling support for a single spatial stream even though the chip they were using supported 2 (it's very common for system designers to choose not to fully exploit every possible feature of a chip they're using, when the tradeoffs like saving battery and size are good enough).

The router might support a 300 Mbps link speed but your client devices don't.

The advertised maximum requires 40 MHz channel width (twice the capacity of regular 20 MHz channels) and two simultaneous MIMO streams (twice the capacity of a single stream).

If your client device supports neither MIMO nor 40 MHz channels, naturally you'll only get a quarter of that 300 Mbps.

(Actually it's very common for Wi-Fi access points themselves to start in "20 MHz only" mode unless told otherwise – wide channels are fine on 5 GHz, but there's just very little space for them in 2.4 GHz band which already only has ~60 MHz worth of space.)

In 802.11n you have a lot of rates (so called MCSes). AP and client sta trying to choose the best rate. Here is full list. https://en.wikipedia.org/wiki/IEEE_802.11n-2009#Data_rates

72 means you are using MCS # 7 because of:

• You have 1 spatial stream (one physcial antenna on device)
• You use best modulation possible (thats because no serious interference)
• You use 20Mhz channel. Thats because there are a lot of other wifi networks so you do not have enough space for 40Mhz channel. When using 2.4 it is almost never possible to use 40Mhz unless you live in woods). To use wide channel try to move to 5Ghz but your AP and client should support it and 5 works bad on long distance.
• You use 400ns (short) guard internal because you do not have serious delays on your line.

So, to increase bandwidth you need client with 2 antennas. You would not be able to get 300 (because you can't use 40Mhz in 2.4) but 144 is possible.