How to assign a pll clock to the dual-regional clock network

No, not necessarily. It could mean that if those clock networks were used, the design would not be able to meet other requirements, such as meeting your timing requirements.

If you are seeing timing failures, you should investigate what those are related to and fix them.

Basically, if it ain't broke, don't fix it. If your design is working fine without using the dual regional clock, then there's no need to try to use them.