If port forwarding is possible
From the information you've shared, it sounds like they may be handing you a dynamic IP or some sort of configuration that is ever changing. Although PIA supports port forwarding as you've said, that doesn't help if the users do not know where the server lives (your VPN IP address).
In this scenario, simply have UDP 9987 forwarded as usual, but you may want to use a dynamic IP service such as No-IP. They have clients that can update the DNS entry yourhost.no-ip.tld from your machine that is running the VPN and Teamspeak 3 server.
On a side-note, to eliminate any strange behavior it would be generally advised to use the OpenVPN client if possible. This permits you to see what kind of configuration they are enforcing to a reasonable extent.
If port forwarding is truly not possible
You might be SOL for running this through this particular VPN service, as from what I've been able to uncover, their system to permit port forwarding is very strange and hit-and-miss from user experiences.
There are potentially options for getting this to work still, such as inquiring with PIA as to whether or not they permit split-tunneling, for which some data can go through the VPN interface, while others go out your hardware/normally expected interface.
Despite these potential workarounds, I would highly recommend simply using a VPN provider that actually supports port forwarding without a huge hassle to save yourself from this headache. I personally have used nVpn.net in the past, which permits port forwarding (with no hassle!) despite handing out dynamic IPs.
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