You want a DSL modem with IPv6 capability and CenturyLink Level 3 support. See the link at the lower right of »
internethelp.centurylink ··· ems.htmlThe C1000Z modem/router is an ADSL2+/VDLS2 modem. I like this modem because you can monitor its operation using DSLstats, a free utility program available at »
www.s446074245.websiteho ··· e.co.uk/DSLstats cannot monitor the CenturyLink PK5001A or PK5001Z ADSL2+ modem/routers. You can find those modems on eBay for $35 or less.
The major factor limiting CenturyLink DSL performance is the distance between your modem and CenturyLink's modem (sometimes referred to as a DSLAM). DSL signal quality is better and capable of supporting higher speeds as you get closer to CenturyLink's facility.
Your older TP-Link modem/router may not support the newer and faster ADSL2/ADSL2+ standards. If not, a new modem may allow higher speeds. All ADSL2+ modems will provide similar performance. A modem capable of VDSL2 operation could provide speeds higher that ADSL2+ modems provided you are very close to CenturyLink's facility.