[Moto G5S Plus Questions & Answers] rpavery:
TLDR:

Bought a new phone in NZ but I live in US, it lacks all 3g and LTE bands for T-Mobile

Tried flashing stock firmware to overwrite with US variant radio; didn't work, still Asia Pacific (APAC) radio

Is is possible to "change the variant" from XT1805 to XT1806 to get US radio? Or otherwise install US radio version?

If not, does anyone have experience unlocking additional bands with QPST/QXDM on this model?

Finally, can anyone confirm if any custom ROMs contain a radio/modem worth trying?

The details:

While traveling my Nexus 5x bootlooped, so I bought a Moto G5S Plus (XT1805, dual-SIM, 3/32 edition, software repo is retapac) in New Zealand, naively thinking that it would work in the US since it's also sold in the US. Got back from my trip and the phone cannot connect to anything better than 2g (EDGE) (but oddly on layover in San Francisco LTE worked). I'm a T-Mobile customer and found that this phone lacks the key T-Mobile bands UMTS1700/2100 (band 4) for 3g and LTE1900 (Band 2) LTE1700/2100 (Band 4) LTE700 (Band 12) LTE1700/2100 (Band 66) for LTE. Now I'm pretty sure the hardware support is there, I just need to switch the radio to the US variant (XT1806). Of course I can't return the phone now, and Motorola reps were worthless (level one rep insisted no US phones work overseas at all!)

What I've tried:

I unlocked the bootloader and reflashed the latest (SANDERS_NPS26.116-61_cid50_subsidy-DEFAULT_regulatory-DEFAULT_CFC.xml.zip) stock firmware from the firmware center. Upon rebooting, even though the firmware notes indicated a US version, it must somehow see that the phone is XT1805 and flashing the modem and baseband still resulted in APAC radio bands and no LTE signal. I tried also flashing the SANDERS_NPSS26.116-45-5_cid50_subsidy-DEFAULT_regulatory-DEFAULT_CFC version, which worked except the gpt.bin file didn't install since it's an older version of what was already on the phone (validation failed due to not allowing a downgrade). After reboot and setup, I'm still on retapac with no LTE service. None of the firmware versions indicate a specific region, and this long thread leads me to believe the firmware is for all variants, so somehow it recognizes the variant on install.

Other Options

I know it's possible to also root with TWRP and Magisk and then install a custom kernel/ROM and use some special tools (QPST and QXDM) to modify the NVram to unlock additional radio frequency bands via this thread. I'm willing to try this, but I know it's risky and not fully documented for Moto phones (guide is for Sony). In the end, I don't really care about keeping the APAC bands - I'd be happy to just switch the radio bands to the US version.

I've read over 100 web pages and forum threads in the last few days researching this and looking for solutions. I'd much rather find a way to install the US version of the radio/modem and baseband, but if that's impossible, I'd be happy for anyone to share experience with unlocking bands on this phone. Finally, if these are not good options, can anybody confirm a custom ROM for this phone that includes a radio/modem so it doesn't use the existing stock radio/modem?

Thanks and sorry for the long post, just wanted to provide as much information as possible. Thanks!

Ryan