If it was possible to change the provider (while still keeping the same number) the same day, I would have done that. I probably should have spent some time investigating if it was possible to buy a new sim card.

My employer pays the bill and they have chosen this provider. If I choose another provider, it will cause more paperwork to get the bills refunded, and I absolutely hate such paperwork, so I will rather ask my employer to raise the quota and be more observant on the "consumption" and threshold warnings in the future. I could probably have gotten the quota instantly increased if Wednesday had been a completely ordinary working day and I'd been in the office.