You probably don't want to install OpenWrt on that device. Wifi is barely supported and will have absolutely miserable performance. Notice the big info/warning box at the top of the page:
Devices with Broadcom WiFi chipsets have limited OpenWrt supportability (due to limited FLOSS driver availability for Broadcom chips). Consider this when choosing a device to buy, or when deciding to flash OpenWrt on your device because it is listed as supported. See Broadcom WiFi for details.
I have and administer a lot of these older Broadcom "Northstar" devices which are all happily running DDWRT still very good supported and on K4.4. SLTS release, with hardware acceleration (which has its drawbacks), they can even do LAN<>WAN gigabit throughput (Ipv4 only).
See: https://forum.dd-wrt.com/phpBB2/viewtopic.php?t=324087
So if you do need wifi DDWRT might be a good choice.
Thank you very much psherman and egc for your prompt replies. I don't need WiFi in the Buffalo WXR-1900DHPD.
I have some other routers with OpenWrt, and then was thinking in using OpenWisp to have a centralized configuration place (because of security/firewalling/etc, sometimes I have forgotten to set an option in a router which I did set in other routers, so security was not really the same). I know it can be done through ssh instead of OpenWisp. I don't seem to see a DD-WRT backend in OpenWisp, although it doesn't seem difficult (??), because there have been tried OpenWisp django (??) backends (https://www.reddit.com/r/DDWRT/comments/j8bnin/central_controller_for_ddwrt/) for
Raspbian (https://netjsonconfig.openwisp.org/en/raspbian/backends/raspbian.html) and Ubiquiti AirOS 8.3 (https://netjsonconfig.openwisp.org/en/airos/backends/airos.html),so a
backend for OpenWisp to DD-WRT doesn't seem to prevent using DD-WRT for the Buffalo WXR-1900DHPD.
Thanks for your patience and for your prompt replies.