Get-RemoteMailbox: This cmdlet is available only in on-premises Exchange.
Get-Mailbox: The Get-Mailbox works for both on-prem and On-Line versions of Exchange. However, it's preferred to us Get-EXOMailbox for Exchange On-Line version.
The Get-RemoteMailbox and Get-Mailbox cmdlets are found in the ExchangePowerShell module. Is that module loaded in your PowerShell session?