[Solved] D-Link DWR-921 HW C3 WAN interface very slow

Hi!

I turn to you with the following problem: If I install OpenWRT on the router named in the address, the download speed available on the WAN port is only 10 Mbps. If I turn on either the software or hardware flow offloading feature on it, my download will be slower (about 7-8 Mbps). This router works by default after a fresh installation without touching any features. If I put the factory software back on it, the download will be 92 Mbps. I have a 500/20 Mbps internet subscription at home, but the ports on the above router know 100 Mbps. My upload speed reaches 20 Mbps in all cases (so with factory software and OpenWRT as well), which means I don't mean a hardware or driver problem, but rather a firewall or software. (By the way, we bought such a router so that if the wired Internet goes away, it will switch to SIM mode soon. The speed of SIM Internet is not relevant in our case.) Because we are using VPN software (Cisco 900- as series), which is handled by the router, so we want to add OpenWRT.
Can you help me where to go, what to look for, what else to try to get a normal speed out of the device, at least a 50-60 Mbps?

Router details:
D-Link DWR-921 Hardware version: C3
OpenWrt 19.07.7 / 21.02.1 (I tried both versions)

In the end, I struggled with it until I was able to resolve it: for some reason the original WAN port can't be handled well by OpenWRT, so I thought one thing and reconfigured the LAN1 port to a WAN port, then it worked fine, so my download speed was 92 Mbps.

That would be rather unexpected, as WAN seems to be just another port on the (SoC-) integrated switch, not a dedicated interface - accordingly is 'should' work just as well/ bad as any of the LAN ports.

Although this hardware is severely underpowered due to its 100 MBit/s switch for a 500/20 MBit/s contract in any case…

Yes, it's pretty weird for me to work that way, but when I put the WAN port between the LAN ports out of curiosity (by configuring vlan), it was slow. Physically, by the way, the WAN port itself is soldered separately to the device, see https://openwrt.org/_media/media/dlink/dwr-921/dlink_drw921c3_inside2.jpg , but as I see it, in principle a chip drives everything... In any case, this is how it works, I only need two ports from it, because then the switch splits the network...

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