You're from Minnesota?

Sounds like the att uverse service, basically they run fiber to a node then from there bring it in on the phone line, the fact they can get 50mbs is kinda amazing since most cat3 rated phone cable had a design limit of 10mbs, but through pair bonding they can get more. A true fiber internet would be fiber into you home, they install a node ( ciena or other brand ) then give you a ethernet connection hand-off from there. I just signed up for att 1000 fiber and cant wait.
As for your line cord connection, is you were extending it over 25ft you may run into crosstalk issues, it still only uses 1 pair ( two wires) and and silver satin rg11 cord will do fine. The extra wires are for phone lines, so if you have say a 3 line analog phone then you would need a 6 pin cord but for dsl the extra pins do nothing. If you really want to play safe when using a longer cord try and find one that is twisted pair, not to common but they are out there. Luckily i work in this field so i havent bought a cord in 20 years since i can just make what i need lol

If you rent a modem NEVER expect them to send you an upgrade. Every about two years you should see if a new model is out. Also I recommend twice a year power cycling a rented modem and looking for new router firmware. My girlfriends parents where having problems streaming Netflix and got talked into faster internet service but they still had problems. Nobody told them the modem they rented was 8 years old and the original reason they had problems. I got it changed out and now they have one with built in Wireless AC the old one was only Wireless G to give you an idea of how old it was.

are you sure your running a fiber to the home (ftth) line cause if it is in fact a telephone cord (rj11) and not the (Rj45) network cable  you are 100% on a fiber to the nod (fttn) line witch means the fiber would stop somewhere in your neighborhood like 500m or 547 yard and continue to your home via copper i would find that out before going any further