fwiw, try increasing the timeout ?
I recall 3 years ago when another LEDE 17 user was witnessing PPPoE disconnections and solved it with these settings
> option keepalive 100 30
>
> I may be mistaken, but above values would offer a 3,000 second (50 hours)
> timeout before router would forcibly disconnect PPP session.
Have you tried 19.07.2 or something older like 17.01.6 or 18.06.4 ?