Great to hear it is solved.
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Thanks! 
About the speed, that is lower than expected, could be because Broadcom is closed source so not well supported on OpenWRT, see the warning on the wiki: https://openwrt.org/toh/hwdata/asus/asus_rt-ac56u
I have and administer still a lot of these Broadcom routers e.g. Netgear R6400, R7000, Asus AC68U, Linksys EA6900 those do about 25-35 Mb/s on OpenVPN and about 100 Mb/s using WireGuard and are still well supported on DDWRT which uses K4.4 which is still supported as SLTS release.
Those have hardware offloading so can do about 900 Mb/s LAN<>WAN throughput for IPv4. I think those routers are also still supported by Fresh Tomato
If you really want to have those older Broadcom routers in operation you might have to look at other firmware then OpenWRT 