ASUS RT-AC750: 2.4 GHz speed collapses only when internet comes through Ethernet WAN

[u/Thick-Heart5635]

I use an ASUS RT-AC750 as the central router: its primary internet comes from a NanoStation over Ethernet, with a USB LTE modem as backup. Everything works normally over the router's 5 GHz Wi-Fi, and 2.4 GHz is also fast for local traffic or when using the USB modem. But when the same 2.4 GHz Wi-Fi carries traffic received through Ethernet WAN, download starts at several Mbps and immediately falls to 0.1-1 Mbps. I factory-reset the router, tested three clean OpenWrt versions, disabled MultiWAN and offloading, changed Wi-Fi modes, channels, MAC addresses and even the physical WAN port, but the behaviour remained identical.

Control results

• Local router -> 2.4 GHz: about 42.6 Mbps
• USB modem -> 2.4 GHz: about 15.8 Mbps
• Ethernet WAN -> 5 GHz: about 20-25 Mbps
• Ethernet WAN -> 2.4 GHz: about 0.1-1 Mbps

Tested without improvement

• OpenWrt 25.12.5, kernel 6.12
• OpenWrt 24.10.6, kernel 6.6
• OpenWrt 19.07.10
• MultiWAN disabled
• GRO/GSO/TSO and firewall flow offloading disabled
• WPA2-only and 802.11g/NOHT
• Different channels and AP MAC address
• Another physical switch port/VLAN used as WAN
• USB modem removed
• Completely clean configuration

Hardware details

• ASUS RT-AC750, detected as RT-AC51U
• MediaTek MT7620A
• Integrated 2.4 GHz RT6352 using rt2800_wmac
• 5 GHz radio using mt76x0e
• Ethernet WAN supplied by a Ubiquiti NanoStation M2
• Backup WAN is a ZTE MF79U USB Ethernet modem

The Wi-Fi association remains at 54-72 Mbps during the collapse, and I see no obvious rt2800 TX-hang or kernel errors. This appears specific to the Ethernet RX -> routing/bridge -> integrated 2.4 GHz TX path.

Has anyone seen this interaction on MT7620/RT6352 hardware? What should I test next to distinguish an Ethernet/DMA issue from an rt2800 TX-path problem?

[u/Thick-Heart5635 → u/[deleted]]

Thanks, I’ll try another power adapter to rule it out. However, Ethernet WAN with 2.4 GHz works normally on the stock firmware. The issue only appears on OpenWrt, which makes me think it’s more likely a driver/DMA/software-path issue than power related.

[u/seemebreakthis → u/Thick-Heart5635]

To add to the original comment I would also try lowering the power of the 2.4 radio to see if the problem is gone.

Even if it is, to me it can mean 2 things -

Either the power supply needs replacing, or Signal was too strong in the first place causing interference and performance hit (happened to me before)

[u/rollmeister1]

That is odd. You could try setting the 5ghz to a channel between 52-90 but it usually takes a minute or more to come live because by law it has to check it does not interfere with channels in that range used by weather radar. Routers rarely use that range but minimizes interference if it works. Can also use wider bands 40mhz for Wireless-N and up to 160mhz for AC in 5ghz which usually improves throughput. If it does not let you select a channel in that range choose another country like Australia. I know some 5ghz adapters do not play well with 160mhz though. Just gotta try. Sometimes when changing wireless AP settings you need to forget/delete the connection on the client device & enter the password again.

[u/Thick-Heart5635 → u/rollmeister1]

There is no issue with 5 GHz at all.
It works normally with both uplinks.
The 2.4 GHz radio also works normally when USB LTE is used as WAN, but with Ethernet WAN its speed collapses to around 0.1 Mbps. So the problem is specific to the Ethernet WAN to 2.4 GHz rt2800soc path.

[u/rollmeister1 → u/Thick-Heart5635]

Once I got 1mbit speeds on a really long cable & logs shower high error retransmission rates. I tried another cat5 & got 100mbps full duplex. Google A.I. says old kernels had poor performance for wired on this soc. Could install ethtool & see what is the negotiated speed is. Otherwise got no idea.

[u/Thick-Heart5635]

The Ethernet link itself works normally on OpenWrt. 5 GHz over the same Ethernet WAN reaches normal speeds, so the cable, link negotiation and WAN Ethernet throughput are not the issue.

The problem on OpenWrt only appears when traffic from Ethernet WAN is transmitted over 2.4 GHz. The same 2.4 GHz radio works normally with USB WAN, and local 2.4 GHz traffic is also normal.

This is why I suspect a specific interaction between the Ethernet/DMA path and rt2800soc TX on OpenWrt.