Dynamic DNS update should work for the WAN fail-over from Primary WAN to the backup WAN or recover from backup WAN to the primary WAN (Primary WAN is Active and Backup WAN is in standby) .
Please check the simulation logs below:
Aug 08 10:01:56 DDNS: Domain mypeplink.myftp.info updated successfully for WAN 1
Aug 08 10:01:54 WAN: Mobile Internet standby (10.248.4.201)
Aug 08 10:01:54 WAN: WAN 1 connected (192.168.52.27)
Aug 08 10:01:12 DDNS: Domain mypeplink.myftp.info updated successfully for Mobile Internet
Aug 08 10:01:11 WAN: Mobile Internet connected to Celcom (10.248.4.201)
Aug 08 10:01:11 WAN: WAN 1 disconnected (WAN failed PING test)
Aug 08 10:01:09 DDNS: Domain mypeplink.myftp.info updated successfully for WAN 1
Aug 08 10:01:03 System: Changes applied
Aug 08 09:59:20 DDNS: Domain mypeplink.myftp.info updated successfully for WAN 1
Aug 08 09:59:19 WAN: Mobile Internet standby (10.248.4.201)
Aug 08 09:59:14 System: Changes applied
Aug 08 09:58:18 DDNS: Domain mypeplink.myftp.info updated successfully for WAN 1
Aug 08 09:58:12 System: Changes applied
Aug 08 09:58:08 WAN: Mobile Internet disconnected (Cold standby)
Aug 08 09:58:06 DDNS: Domain mypeplink.myftp.info updated successfully for WAN 1
Aug 08 09:58:04 WAN: WAN 1 connected (192.168.52.27)
Aug 08 09:56:09 DDNS: Domain mypeplink.myftp.info updated successfully for Mobile Internet
Aug 08 09:56:08 WAN: Mobile Internet connected to Celcom (10.247.166.177)
Aug 08 09:55:50 WAN: WAN 1 disconnected (WAN failed PING test)
Aug 08 09:55:46 DDNS: Domain mypeplink.myftp.info updated successfully for WAN 1
Aug 08 09:55:41 System: Changes applied
Aug 08 09:54:38 WAN: Mobile Internet disconnected (Cold standby)
Aug 08 09:54:31 DDNS: Domain mypeplink.myftp.info updated successfully for WAN 1
Aug 08 09:54:29 WAN: WAN 1 connected (192.168.52.27)
Aug 08 09:53:39 DDNS: Domain mypeplink.myftp.info updated successfully for Mobile Internet
Aug 08 09:53:37 WAN: Mobile Internet connected to Celcom (10.247.110.58)
Do you find any logs indicate that the dyndns.org update fail for your device ?
@Rick-DC The shared forum link is more to feature request for all active WAN sharing the same dyndns host/domain name. I think the reported issue by @cover is not related to the link.