I've been dealing with a similar problem on and off for the past month. Randomly, it is impossible to connect to SL and even access to most secondlife URLs (sometimes it drops while connected). When I do a traceroute to SL servers it systematically shows a timeout between these nodes : level3.par04.atlas.cogentco.com and LINDEN-RESE.bar1.Phoenix1.Level3.net
I've tried using different DNS but it doesn't seem to change anything.
Oddly enough, when this happens I'm still able to connect to SL with Lumiya using my ISP LTE network (my ISP provides fiber and LTE). The traceroute shows a different path through LTE though.
My bet is the ISP is to blame here, as when these blackouts occur, I can't connect to Steam either.