This was working, now it's not.

The TP-Link ER7206 router is using "Custom DNS" with the Update URL:
https://api.dynu.com/nic/update?username=[USERNAME]&password=[PASSWORD]&hostname=[DOMAIN]

There are 2 WAN ports (WAN and WAN/LAN3) that are configured as load balancing between two different ISP (Charter & Frontier).

But, the "Custom DNS" profile only allows one or the other, but not both and I've been using Charter.
About 2 months ago SNMP traps saying the update failed with "badauth" started coming in.
Today, I tried changing from Charter to Frontier and it is still "badauth". I'm back to Charter and no luck there either.

I added a new API key, and that didn't help.

Any clues?