5g or 4g

Yes

He’s asking if the 4g will work in a 5g phone. I’m working through this very same issue right now. Same SIM card from my XR, went Into the 12max Pro. Where there was once 18meg down, 6 meg up, NOW is .5 meg down, .2meg up.

Im hearing that this a common problem at the moment...

Tried that as per tech help three times today, to no avail...

Yes, AND no. Yes, the general cellular voice will work, but data will be SLoW! When you send a text, it will retrieve the name of the outgoing text slow....

im on straight talk, and this just started to happen....

APN settings are good, permomed a few network resets, and Apple ran a Diag on the phone with no errors.

In short, get a 5g plan for an5g phone....

Your problem is unrelated. 4G and 5G are comparable in data speeds in most locations. In some 4G will be faster, in others 5G will be, based on the distance to the tower. The ultra fast 5G is only available in a few dozen locations in the US currently, and it is very short range (comparable to Wi-Fi in range).

Bythelake2017 wrote:

Same SIM card from my XR, went Into the 12max Pro.

That right there might be your problem, try a new SIM card. I have a 12 Pro on Verizon, without a "5G Plan" and have no such problems.

Tech support at both ends have found no problems. Apple found no hardware/software faults, and Straightalk tech support says NO changes have been made to any APN settings for the new phone. They sell the 12max as well.

im leaning towards a new sim, and port over....

You might try Settings> General> Reset> Reset Network Settings. You won't lose any data, but your iPhone will forget previously known Wi-Fi networks, and perhaps some Bluetooth devices, and your iPhone name will revert back to "iPhone" if you had changed it. But this may help?

Another thing is Settings> Cellular> Cellular Data Options> Voice & Data ... change from 5G On or 5G Auto to just LTE and see if that helps. This disables 5G, but may help LTE performance? Not much 'real' 5G anyway LOL!