go here:https://www.sammobile.com/firmwares/
and make sure you enter the exact model of your phone because firmware is very device specific down to the exact model. instructions on how to flash the firmware is given on the website. basically you will need a computer and a program called odin to flash the firmware.

it has always been this way. not sure what you mean the the software has changed. where did you get the firmware from?

 

you have an s9 so the standards then still applies now because your device is pretty old by phone standards. odin can be finicky. i would start with the most current one and you can then try older versions until you find one that works.

what is your build number?

 

no the pda does not match your build number

use this one:
https://www.sammobile.com/samsung/galaxy-s9/firmware/SM-G960U/TMK/download/G960USQS9FUG2/1568009/

 

again it is device specific. if the phone is locked in to ATT, then when you do a firmware update, you will get the same thing with all of ATT's bloatware.

you cant just flash any firmware. it has to be made for your device.

and are you sure you have the correct model number?

 

ok nice find.......but in general unless you are 100% sure a firmware that is not for your device works on the phone, i would always flash the correct firmware designed specifically for your phone. i have seen here many threads of people flashing the incorrect firmware with disastrous results.

 

it depends on the carriers as each carrier has a different bands of signals that require the radios to be tuned to. some will use the same frequencies and bandwidths, but others will not. plus some firmwares will include updates to the carrier's or manufacturer's own skin like Samsung's One UI. so updating the wrong firmware could make the phone useless as it will have software that will be trying to use the wrong frequencies or the something that is in either the carrier or manufacturer's updates will screw something up.

not sure on U1, but you might be right.

 

The U is just part of that identifier for that ROM version, it's just part of the assigned name. There no such thing as a 'stock Samsung version before branding', nor is there any such thing as a universal stock ROM that anybody can just download to flash on their phone. If you want to install a version of Android on your S9 that's not preloaded with all that system-level bloat, you'll be better off looking into flashing a third-party ROM, like Lineage OS.

 

Again, just to repeat, there is no such thing as a stock Samsung ROM without Samsung's branding. Nor is there any such thing as just a stock Android ROM that's universally compatible with any Android device. Where are you reading that there are stock Samsung ROMs that don't include Samsung branding? Does not sound like a very valid news source,

In your situation, you have a phone with a) Samsung bloatware and b) AT&T bloatware. All that cruft is preinstalled and integral to all the stock ROMs that match your phone model. And that's the part you need to pay attention to, stop focusing on using any 'stock' ROM.
You need to look elsewhere at one of the third-party, custom ROM projects:
https://www.getdroidtips.com/custom-rom-galaxy-s9-s9-plus/

 

Hotspot support is already integral to the Android operating system itself. It's not so much what phone you're using nor which version of Android it's running but more a matter of your account with your carrier. Some carriers include hotspotting for no extra charge, some require an added charge, but you need to talk to your carrier.
https://www.att.com/support/article/wireless/KM1009376/
This does reinforce what I was pointing out earlier, this whole matter where you're so fixated on a U1 ROM as a magic fix is very misguided. Whatever this source is that you keep reading is apparently more about trolling and feeding its readers fake concepts and misleading suggestions as some kind of sick joke.