I'm on HFC NBN 100/40 using the 5366TN router. I seem to have an issue with the 5GHZ band, it seems to sometimes connect at very low speeds, e.g. 21Mbps, which means speedtest will give me 3-4Mbps download but will gives 35+ upload. Can anybody tell me why this is happening? Most of the times it will connect at higher speeds e.g. 433Mbps, which will give 90+ download speed. This is seen on the homepage of the router where for 5GHz devices it will tell you connection speed, when you click on the device it will give more attributes such as signal strength, IP MAC etc.
You have enviable performance from your NBN HFC connection.
I have found speedtest.net to be a highly variable at times and have treated its results as indicative samples vs accurate. The 3-4Mbps Down /35 Mbps Up is a very unusual result which doesn't initially make sense, unless during the testing, the device and modem swapped from a 2.4Ghz SSID used in Downlink to the 5GHz SSID for the Uplink section of the test. I have found doing consecutive testing with near servers to show varying speeds.
I don't have a clear answer, but this may give you some understanding of possibilities. The 21 Mbps speed may imply a 2.4Ghz connection was initiated for the test and could have occurred due to a clause 3 condition below.
Many factors influence the Wi-Fi connection speed from your device/PC to the modem and it can vary due to
1. location distance of device from the modem and its propensity to relocate when in use within the home. Speed will drop as the device is located further away from the modem.
2. the device's Wi-Fi Adapter antennae 1x1 or 2x2, its Properties and Settings. If you look into the Network & Adapter Setting , choose Wi-Fi Adapter, you should see the speed it is currently connecting to your modem and this may change as you monitor it even when the device is stationary (close , then open up the window again).
3. the modem's Band Steering capabilities and how they are handled by the device's adapter; how it monitors both 2.4 Ghz and 5 Ghz SSIDs and dynamically manages connection of the device between the two SSIDs depending on its rules. Some devices/modems will always connect to 5GHz disregarding the 2.4Ghz or lower the power on the 2.4 Ghz so that its signal is not as strong, others will swap over to the slower 2.4Ghz if signal weakens beyond a set limit on the 5Ghz, or if too many 5Ghz devices are in use, it may swing one over to the underutilised 2.4Ghz.
4. I don't think there is a common Standard for this, how each modem and device does band steering is generally not disclosed by the manufacturers; if the modem and adapter are made by the same vendor, things work well. You may see both SSIDs in your Network Settings and have them registered using the same or different SSID names and passwords. Some legacy devices can't see a 2.4Ghz SSID if band steering is enabled in the modem and if that is the case, the solution is generally to disable band steering if possible.
5. When Band Steering is enabled in the modem, the number of abundant 5Ghz channels may be reduced to non DFS Channels thereby limiting the number of channels that 5Ghz devices can link to for channel bonding which is required for higher speeds.