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