Since modern sites are connected to Office 365 groups, modern site group membership is managed through the Add-UnifiedGroupLinks cmdlet and takes a bit more effort. If you need to do it (for example, performing the activity in bulk), you may want to administer it via PowerShell with this process:

1. Connect to Exchange Online PowerShell:

$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential (Get-Credential) -Authentication Basic -AllowRedirection;
Import-PSSession $Session

2. Find the appropriate Office 365 group using the SharePoint site URL as a filter:

$Group = Get-UnifiedGroup -ResultSize Unlimited | ? { $_.SharePointSiteUrl -eq "https://<tenant>.sharepoint.com/sites/<site>"}

3. Add the new owner to the Unified Group Links Members attribute:

Add-UnifiedGroupLinks -Identity $Group.DistinguishedName -LinkType Members -Links <[email protected]>

4. Add the new owner to the Unified Group Links Owners attribute:

Add-UnifiedGroupLinks -Identity $Group.DistinguishedName -LinkType Owners -Links <[email protected]>