As of March 2020, a TD-W8970B with a current OpenWRT 19.07.1 image is not bug free and sometimes loses the ADSL connection on its own (https://bugs.openwrt.org/index.php?do=details&task_id=1794&string=8970). But that rarely happens. My internet situation was, that I live in an old building with old wires and old people around me. The internet was very, very slow, Downloading with 100 KB/s was barely possible. The mentioned OpenWRT-TD-W8970B did react badly on that, losing the connection quite often (every hour or so), and then no internet was available at all. A stock firmware FritzBox 7312 gave me the same slow internet, but that one stayed online all the time. So there was some internet there.
Through some miracle, better ineternet came across and now I can surf with proper speeds (more than 1 MB/s). The TD-W8970B is now working well, not losing the connection that often anymore.
I just want to share this experience with OpenWRT. There seems to be a problem with very, very slow connections.