SENSE does not always respond to DNS requests
It look like SENSE does not always respond to DNS requests. I believe this was what was causing issues with my Bosch dishwasher (https://community.f-secure.com/t5/F-Secure-SENSE/Bosch-dishwasher-with-Home/m-p/95771) and now seeing it with the Garming Fenix 5 watch as well.
I have debugged this by dumping all traffic on both sides (the LAN and the Internet) of the SENSE. My watch sends DNS qureies for time-c.nist.gov and gold.garmin.com to SENSE. SENSE sends these queries onward to my Internet router, the Internet router replies with the addreses to SENSE, but the reply is never sent back to my watch by SENSE.
However, the same lookups work from other devices. The root issue may be related to ARP requests sent by SENSE, as I have seen ARP requets from SENSE to the dishwasher and the watch that have gone unanswered. I have not yet investigated thoroughly, but with a quick look at the ARP requets sent by SENSE seem to differ somewhat from requests sent by other devices. Given time I will investigate more thoroughly.