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o2 UK

AbsolutelyRidiculous

ULTIMATE Member
Has anyone had this experience?

iPhone 14/15

o2 sim.
A second sim, eg EE.

o2 will only connect to 4G.
The second sim will connect to 5G no issues.
Turn off the second sim, o2 connects fine to 5G.

What’s going on here?
 
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Using "5G On" instead of "5G Auto" makes iPhones hold on to 5G when they usually would drop to 4G, but I'm not sure if it would help here. You can't force it like you can on Android.

Sometimes it's the device, either lack of support for using 5G on both SIMs at the same time (probably not the case that on those phones) or just to save power (Apple is a bit aggressive here)... The SIM/eSIM that is used for data connects to 5G, aggregates more bands, etc, while the other SIM - only active for calls and texts - uses something that uses less power.

Sometimes it's the network. My O2 SIM doesn't like to connect to a NR band if I'm not using data. Very different from Three, EE, and VF.
 
On iOS I don't really understand how it behaves but it does seem the line not active for mobile data stays on 4G. EE (secondary line with 1p) have 5G where I live and when Vodafone (primary) is selected as data line which is 4G only (still plenty fast enough) the EE line will be only on 4G despite 5G being present. Some networks have different thresholds they switch you to between LTE and NR or in Vodafones case the dreaded EDGE (doesn't seem to happen as often as it used to). EE very rarely will go onto EDGE unless the signal is very weak (only happens in a couple of places in my town). Regarding dual SIM with 5G available on both lines I cannot see why it wouldn't work but I guess switching from 5G Auto to 5G ON may help. The location I had Vodafone 5G was a weird 4G only spot for EE the other week (represented on their coverage map as 4G only).
 
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