fwiw, try increasing the timeout
In LuCI, go to Interfaces -> WAN -> Advanced Settings
You will find two settings ‘LCP echo failure threshold’ and ‘LCP echo interval’. What are the values? Would they be 0 and 5 ?
Try changing the ‘LCP echo failure threshold’ value to say a value of ‘5’. (The default value of 0 never used to work in old LEDE 17 days....)
This was used to fix FTTC disconnections when using PPPoE.