To share the same SIP address in On-prem and online (Teams) required hybrid to set/change Hosting Provider.
for you to run get-cshostingProvider you will required Skype for business Online (old) module or Skype for business ISO mount and install the tools.
Hosting provider identity should be Office365
Proxy FQDN should be "sipfed.online.lync.com"
Enable to $True