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5G comes in different variants of 5G depending on the frequency range:
• Low band (700–900 MHz) → long range, good indoors, but not that fast.
• Mid band → balance between speed and coverage (most common).
• High band (mmWave, 24–40 GHz) → extremely high speed, but short range.
These frequencies are not supported in all countries.
 
The Apple Watch Cellular has different hardware versions for different markets. There are at least two and maybe three variants, each with different modem configurations. I only know this because I moved countries and dived into the reasons I couldn’t get my cellular version to work in my new country. Apparently, the device is too small to hold a modem that can cover all the bases (like the phone can).

So, maybe a regulatory or commercial issue, but could also be a hardware limitation pending development of the required modem.
 
I know in Canada, Bell and Telus currently don't allow non-phones and tablets to access the 5G network. Other IoT devices and smartwatches are limited to LTE.

I don't think this applies to Rogers, though? I'm pretty confident I've seen in the past in promotions that they say smartwatches can use the 5G network if hardware allows. They also did announce at the start of June that they were beginning the rollout of 5G RedCap, but didn't specify where/how long until it's widely available.

This could be a case where the hardware can do it, but carriers need to catch up.
My M4 iPad Pro is connected to 5G+ on Bell Mobility
 
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