So far, all I know are two things:
- You have a Frankensystem. Your kernel is 5.3-release, without any patches, and your rc.conf(5) is 4.1, edited locally, and with an adjustment for changes to rc(8) that were made at 4.2-release.
- Your skills with OpenBSD are limited. I have recommended using rcctl(8) to start isakmpd on your "backup" system, but you have not stated anything about testing this, or the results obtained.
Anything else about your system would be guessing. I only know what you have been able to tell me.
Why don't you try using rcctl(8) to enable the isakmpd daemon on your "backup" gateway, and see if you can get it working?