You can restore a ePO - On-prem server in FIPS mode only if the server was previously running in FIPS mode.
You can't restore a ePO - On-prem server that wasn't in FIPS mode as a FIPS mode ePO - On-prem server. The ePO - On-prem software and database must be reinstalled as a new instance of ePO - On-prem.
The complete ePO - On-prem reinstallation is required because all existing signed and encrypted content was signed with non-FIPS mode keys. Also, the database contains content encrypted with non-FIPS mode keys and can't be decrypted with the FIPS mode keys.