UPDATE:
I changed the VPI/VCI setting on the CenturyLink to "Tagged-201" instead of "Tagged-0" like CenturyLink said 🙄. I connected and got through the setup! Still appears to be losing connection every now and then, though. Every 5 to 10 minutes.
I added 1492 for the MTU (saw that in my CenturyLink settings). Still dropping connection.
I've read about people setting VLAN to 201 on the Netduma, but when I go into WAN Settings > Advanced to do that, the UI lets me toggle on VLAN, but when I type in 201, it does nothing and doesn't stay set after clicking Save. Normally when you tap Save, it goes back to the prior menu. This just stays on that screen and if I exit and go back in, VLAN is still toggled on, but where you put the 201, it's at 0 and says, "Must be between 1 and 4094".
My IPv6 (WAN) setting is toggled on if that helps.