We had a problem where M365 users could not retreive Free/Busy information from on-premise Exchange server. Hybrid setup was run and everything was working fine except this.
After research I found out that TargetSharingEpr is the way to look at. There are many sites out there that are pointing to the same setting but only Set-OrganizationRelationship is used to change the value. The problem is, this value is available for two commands:
While we changed the value in Set-OrganizationRelationship it didn't work until we changed the value in Set-IntraOrganizationConnector too.
By default it should work with just TargetAutodiscoverEpr but for us it didn't.
Finally we ran this command on Intra Organization Connector which is used from Cloud to On-prem:
Set-IntraOrganizationConnector -TargetSharingEpr https://mail.domain.com/ews/exchange.asmx
And it worked.