I'm going to second @np_f1_2021's advice. The microphone you use on a phone call is the lower one, located on the lightning connector flex cable. You can verify this by switching to the top microphone when you put the phone on speakerphone mode.
Anyway, yes changing the charging port flex cable is the way to go to fix a bad microphone. You might try cleaning out the grille on the bottom of the phone to make sure it's not just clogged up with pocket lint or something; a toothbrush, rubbing alcohol and some compressed air should do the job.
If that doesn't help, here's the guide you'll need to swap out that lightning port assembly.
iPhone 11 Lightning Connector Assembly Replacement - iFixit Repair Guide
iFixit only has a video guide available for that repair, but if you prefer a step by step written guide, the Spanish site Nadie Me Llama Gallina has what you want.
Manuales / Apple iPhone 11 / Flex with charging connector
The part comes in different colors to match your phone color, but in practice the color won't affect the operation.
iPhone 11 Lightning Connector Assembly
Be sure to grab a sheet or two of the precut display adhesive to seal the phone back up when you're done. I usually buy a couple just because they're easy to mess up; if you don't get it on just right the first time you don't get a second chance - instead you'll end up with a ball of glue. So as to not have to wait for a reorder to arrive, I buy a couple to be safe.
That should get you going again; let us know what you find and how it goes!
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