Did you manage to successfully update?
Unfortunately not. I had a go following the instructions in the mega link but the AT commands to update would always fail. Not surprising really as it seems to verify you are on the correct vendor and version of the firmware before flashing, so the supplied Telia OTA updates were not likely to be compatible with the Three branded stuff on my device.
From reading the eko.one forums the path to a fully updated mini-PCIe card seems to be flashing the whole modem with Telia or Elsia ISP firmware using what they refer to (or google translate does) as the 'framework', which I think is one of those 500+MB mega packages with windows binaries in, then incrementally pushing updates that have been extracted from ZTE OTA updates, there is this python script for pulling the 'delta' updates using a random IMEI:
https://gist.github.com/Leo-PL/051a0b2ba26c3495afa47746e4a809cdBut I cant get it to find any Three related ones.
I dont have a link but some other discussion on that forum led me to believe that setting a locked cell didn't always allow for carrier aggregation, or that it may be dependant on network provider. This coupled with moving the device to a different location that gets roughly 10mpbs upload on band 1 has removed motivation for trying to get the modem updated.
Would still like to put Openwrt on it, but as you say, in bridge mode it just works.