Static IPs disappear from outside gateway
I recently switched to a block of 5 static IPs. I have one major issue: Anywhere from 15 minutes to one hour, the statics assigned to individual servers become inaccessible from outside the gateway. Behind the gateway, everything is fine. The servers are still able to access Internet resources themselves, and the static show as being assigned internally. Domain names continue to resolve--even newly assigned domain names resolve even if DNS record is updated during the blackout.
All that is needed to re-establish the connection to the servers is to refresh the network configuration on the server (either of the two) OR access the Comcast gateway box and refresh the attached devices table.
I'm not the only customer who has dealt with this challenge. There are a few other posts related, but no obvious solution (except perhaps a cron job to refresh interfaces or something--not an ideal solution).
Servers are Ubuntu 18.104.22.168.
Gateway is the standard business-class model.
There isn't anything sophisticated or unusual about the setup. It's a home office I where test apps. Two ironies:
1) a windows server test machine I borrowed worked flawlessly for nine hours.
2) The cheap home wireless router I use holds a static I threw in there for testing.
I've troubleshot this down to the gateway/server combination. I've opened ports, closed ports, blocked, unblocked, and checked and unchecked every conceivable option. I can safely rule out every possible hardware malfunction on my end.
So, my questions are: Does anyone have any suggestions? What would cause Ubuntu/comcast gateway to unassociate?
Thanks for any help.