man pppd
насколько я понял, за это отвечают параметры lcp-echo-failure n
If this option is given, pppd will presume the peer to be dead if n LCP echo-requests are
sent without receiving a valid LCP echo-reply. If this happens, pppd will terminate the
connection. Use of this option requires a non-zero value for the lcp-echo-interval
parameter. This option can be used to enable pppd to terminate after the physical con-
nection has been broken (e.g., the modem has hung up) in situations where no hardware
modem control lines are available.
lcp-echo-interval n
If this option is given, pppd will send an LCP echo-request frame to the peer every n
seconds. Normally the peer should respond to the echo-request by sending an echo-reply.
This option can be used with the lcp-echo-failure option to detect that the peer is no
longer connected.
у меня выставлено так, и схожих проблем не наблюдал:
lcp-echo-failure 3
lcp-echo-interval 5