I went from version 7.5.14 to 7.6.7 and lost the connection to the OPC.
Could not open secure channel to opc.tcp://ip:port/isopvus/Basic128Rsa15
I don't know what is possible with those versions, but upgrading from one ancient and unsupported old version to another ancient and unsupported version seems unwise.
Consider re-installing v7.5.14 to get running again, then start planning an upgrade to v8.1 via a fully-testable development environment.
If planning the latter, IA support will be able to help. (They might help anyways, but those are both really old versions.)
Is it really possible to upgrade from an older version of 7.5.14 to a newer one like 8.1? Because I tried to do an upgrade from 7.1.8 to 8.1 and it was not possible so I made the decision to do progressive upgrades.
It is supposed to be possible.
But I have seen cases where format changes made it necessary to hop through the last of each LTS version. (Skip 7.6.x, 7.8.x, 8.0.x.)
But I wouldn't try to get external connections working in the interim versions, just make sure everything inside Ignition looks present.
Get connections working only once you reach a supported version.