https://wccftech.com/iphone-12-pro-dual-sim-mode-5g-gets-disabled/

Both the iPhone 12 and iPhone 12 Pro are able to run in dual-SIM mode, but there are going to be some limitations. Dual-SIM mode is convenient for when you want to separate personal and work numbers but if you’re buying an iPhone 12 and iPhone 12 Pro to run either models in dual-SIM mode and experience 5G speeds at the same time, then we have to be the bearer of bad news. According to Apple Sales FAQ shared with retailers and carriers, the ‌iPhone 12 or iPhone 12 Pro‌ will not support 5G on either line when the dual-SIM mode is active.

If customers disable one SIM and start using the other, then they’ll be able to experience 5G speeds. According to MacRumors, this isn’t going to be the case if the owner starts using both SIMs in unison. This will certainly come as a massive blow to customers since the convenience of having multiple numbers in a single phone was unparalleled. Then again, those other devices weren’t providing support for 5G connectivity either.