This same issue comes up every time a new iPhone is released, and it occurs for the same reason:
Cellular carriers need to properly optimize their towers and antennas for the latest cellular frequencies.
When it comes to 5G, the iPhone 12 supports the following frequency bands:
- 5G NR (Bands n1, n2, n3, n5, n7, n8, n12, n20, n25, n28, n38, n40, n41, n66, n71, n77, n78, n79)
- 5G NR mmWave (Bands n260, n261)
The iPhone 14 Pro supports the following frequency bands (differences from iPhone 12 in bold):
- 5G NR (Bands n1, n2, n3, n5, n7, n8, n12, n14, n20, n25, n26, n28, n29, n30, n38, n40, n41, n48, n53, n66, n70, n71, n77, n78 n79)
- 5G NR mmWave (Bands n258, n260, n261)
When a cell phone connects to a cell tower, it tells the tower "Hi, I'm this kind of phone, and I support these frequency bands."
The tower says "Great, let's see what's least congested… OK, use frequency band (as an example) n48."
If the cellular tower isn't properly adjusted/optimized for the assigned frequency band (again as an example) n48, your iPhone 14 Pro may see little to no signal where your iPhone 12 on frequency band n28 sees full bars.
If the tower assigned your iPhone 14 Pro to use band n28 as well, everything would work as it did before, but because the newer frequencies are most likely to be less crowded, that probably won't happen.
Long story short, contact your cellular provider, only they can fix this.