2016-18MY Legacy/Outback 2016-18MY Impreza 2016-18MY Crosstrek 2016-18MY Forester 2017-18MY WRX
See https://static.nhtsa.gov/odi/tsbs/2021/MC-10189606-0001.pdf for details. And, yes, Subaru is reimbursing dealers for this work.
BMW modules only supported 3G (they were older though).
Anything could support the minimum needed to connect to an LTE network with the right software update, but modem chipset makers don't make any money rewriting old firmware for old chipsets, so they refuse to rewrite the firmware to support lte.
Now knowing this, could we (theoretically) update the firmware of an iPhone 3G so it works on a 4G network?
If that's the case (as is common), then there is really nothing to update, short of scraping off that chip or module and replacing the hardware.
I know the 3rd party module makers do tailor their modules to regions and the 'world wide' models can be quite pricey. On the region specific versions they don't include some of the hardware to physically operate on frequencies they don't specify. Sounds crazy, but a few cents here or there is worth running multiple SKU's.
There are probably other issues such as radio compliance and patents that also prevent an upgrade.
The VoLTE upgrade isn't so surprising. For a long time we had 3G/4G modules however the mobile networks didn't support VoLTE so everything would fall back to 3G. Once everyone got their act together, a quick firmware upgrade and VoLTE worked perfectly without adjusting the hardware.