If you have a static IP address and the provider supports it, have them send calls directly to your PBX, instead of registration.
If you must use registration, set a short expiry interval if the provider tolerates it, e.g. 120 seconds, to keep the NAT association open and quickly regain registration should it be lost.
If the provider insists on a long registration interval, (in your hardware firewall) forward the SIP port from their IP addresses to your PBX.