Like most of ASUS settings compared to say a Netgear option, you get many more (options) to configure for a given scenario.
lcp-echo-adaptive :
When adaptive LCP echo is enabled, LCP echo requests are only sent if the link is idle (no traffic) , avoiding the common situation where a congested PPP link (e.g. during torrenting) is falsely detected as disconnected because the LCP replies are not received in time.
LCP echo requests :
An LCP Echo Request are sent once every (x) seconds imposes minimal overhead, but allows for rapid detection of connection failure. One or two consecutive failures will happen from time to time with such a short interval, but 4 in a row almost guarantees the connection has failed.
In simple English – PPP Echo will send an LCP echo-request frame to the peer every ‘n’ seconds. Under Linux, the echo-request is sent when no packets have been received from the peer for (n) seconds. Normally the peer should respond to the echo-request by sending an echo-reply.
If Internet Detection is set to disabled and DSL link drops, you will lose DSL connectivity and access to internet will be lost (always keep Internet Detection Enabled/PPP Echo). I believe (never tested this myself)
Just A good way to help diagnose/verify if you have a stable DSL connection but can be troublesome under heavy load hence why ‘lcp-echo-adaptive’ is useful in certain situations 
https://uktechhub.com/forums/topic/dsl-modem-and-router-technical-terms-and-abbreviations-etc/
In a completely sane world, madness is the only freedom (J.G.Ballard).
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