I am still testing, but it seems to be work as a OpenVPN appliance connecting only the LAN port, not the WAN port. It should probably also work with Wireguard, but Brume is already fast with OpenVPN.
I used the following as a guide:
https://forum.dd-wrt.com/phpBB2/viewtopic.php?t=322028&sid=c1f76eff334335f029e87660b0f480be
I am able to use NetSetMan (free for personal use) to switch my IP settings between my main Internet router and the Brume VPN router.