Very slow internet speeds with RT-AX86U Pro?

[Rocketboy235]

Getting extremely slow download speeds when running speed tests on the router itself as well as on client devices connected via wifi. Haven't run speed test via ethernet yet but if the built in speed test on the Web UI is also very slow, then maybe no point to running more speed tests. Sometimes it would even fail to run the speed test. At least upload speed is okay. The weird thing is that it's not a consistent slow speed each time, once in a while, the download speed will be fast but most of the time it is slow.

The 2025/8/23 test was on the staging network. The ISP speeds on this current network are 300 mbps down and 35 mbps up.

Unfortunately this is a remote area and I have someone else deploy the routers for me so I cannot do on site troubleshooting/testing. Currently connected via VPN to configure the router myself.

I have a RT-AX86U Pro running latest merlin fw and have RT-AX86U as well as AC68U as AiMesh nodes. The AC68U is on it's way out to be replaced but couldn't bring the router to the remote area this time around (next time though).
I tested the AX86U Pro and AX86U on a separate network (my staging network) before having these set up today and they worked perfectly on the previous network.

I am replacing the two AX92U routers with the AX86U Pro and AX86U and didn't have this really slow internet download speed issue before.

The modem I am running is the SB8200. No issues with the modem that I'm aware of.

Tried rebooting the modem and router but unfortunately the slow download issue persists.

I at first thought maybe the many client devices were causing an issue on the WiFi so I tried to mess with the settings for the WiFi but that had no effect which I'm not suprised when I later realized it was the speed coming from the WAN.
Anyone experience something like this before or may have an idea how I can troubleshoot this? Thank you!

 

[bbunge]

If you are testing via WIFI, remember that the AC68U will limit the WIFI capability of the AX86U Pro and the AX86U when it is incorporated into an AiMesh.

 

[OzarkEdge]

Anyone experience something like this before or may have an idea how I can troubleshoot this? Thank you!

Given:

o no issue with the new equipment when setup on your staging network;

o the new network WAN to modem connection status is green/full port speed;

o the new network router webUI speedtest should easily report ISP speeds 300/35Mbps, and should complete/repeat reliably (did it on your network?);

o the 'it worked before' observation is inconclusive... did you speedtest the old network today before upgrading equipment today?

My first thought is to suspect a WAN issue (modem/cabling/ISP), perhaps just for today(?), and try to prove that... me, I'd wire my laptop to the modem and perform a speedtest.

OE

 

[Rocketboy235]

If you are testing via WIFI, remember that the AC68U will limit the WIFI capability of the AX86U Pro and the AX86U when it is incorporated into an AiMesh.

Yeah, unfortunately this is something that will have to be tolerated for another month until the remaining router can be delivered to the remote location.

Given:

o no issue with the new equipment when setup on your staging network;

o the new network WAN to modem connection status is green/full port speed;

o the new network router webUI speedtest should easily report ISP speeds 300/35Mbps, and should complete/repeat reliably (did it on your network?);

o the 'it worked before' observation is inconclusive... did you speedtest the old network today before upgrading equipment today?

My first thought is to suspect a WAN issue (modem/cabling/ISP), perhaps just for today(?), and try to prove that... me, I'd wire my laptop to the modem and perform a speedtest.

OE

Good points OE. I forgot about testing the modem directly. I should have my helper try connecting to the modem directly and let me know how the speeds are just to rule out a modem issue and cable issue.
The built in router speed test is not consistent so there's definitely something going on and I should have done some baseline tests before installing the new router system in (will have to make sure to keep this in mind the next time I do upgrades...)

 

[degrub]

Don't use the speed test on the router if you want a better result. It is loading the CPU. Run the speed test from a LAN cabled PC into a LAN port on the router. Same if you connect directly to the modem.

 

[Rocketboy235]

Don't use the speed test on the router if you want a better result. It is loading the CPU. Run the speed test from a LAN cabled PC into a LAN port on the router. Same if you connect directly to the modem.

I can see where you're coming from though I guess I haven't really noticed that being an issue in the past. Maybe if the router has a weaker CPU or is overburdened with a lot of processes running in the background, I can see that more likely.

In any case though, I do plan to have my helper also run a speed test while having an ethernet cable connected to the router from the laptop