@JasonTwe After you use up all your data, you can't go online except with WIFI. PM offer 4G plan and 3G plan. With 4G (LTE) , data speed is faster than 3G. But talking (voice) service is using 3G not 4G, because PM doesn't have Volte (Voice over LTE) service, I holp PM will offer this service soon.
If your phone is set to actual UMTS/HSPA (3G) then you can see 10 or so Mbps speed but would have some lag with video's and such. If, like most people, the phone is set to LTE (4G), then the speed is set to 3Mbps.
Two other ideas not mentioned beyond buying data adds-ons is asking support to renew early or changing plans immediately to a different plan.
Yes, once out of data then there is no overage or slowed speed. It just stops.