The documentation for PPPoE option keepalive does not seem to be consistent with implementation
# grep keepalive /etc/config/network option keepalive '100 20'
# uci show network.wan.keepalive network.wan.keepalive='100 20'
Observed under Luci
The numbers must be separated by a space. First number is for “seconds between each ping” ( ppp 's lcp-echo-interval option). Second number is “we assume the connection is down after this number of pings failed” ( ppp 's lcp-echo-failure option).