Phone is unlocked from Telus, yet still won't get service on other carriers.

A few years ago, my mom had 3 LG Stylo 3 Plus's, one for her, me, and my dad, she had some issues with Telus, and ended up ending our plan and got all three unlocked in 2019 (the phones were purchased in 2017, when carrier locking was still legal), however, only my dad's got service on other carriers, mine and my mom's didn't. My original Stylo 3 Plus is now dead, but I was given my mom's old one, and I can confirm this is still true, inserting a Bell SIM, it just says emergency calls only. Does anyone have an idea what could be going on? I want to use this as my secondary phone. I also got my dad's Stylo 3 Plus, it's on the exact same firmware, and it worked fine with my Bell SIM, however I can't use that one because it has weird GPU issues, so I'm stuck with two Stylo 3 Plus's that both have issues.

It doesn't prompt for an unlock code when I enter a SIM card, so it is unlocked in some form.

I've considered contacting Telus support, but they'll probably tell me to go to Bell, and then Bell will tell me to go to Telus.

Please do not tell me to buy a new budget phone. I have very little money, and even if I did, I don't want a modern budget phone, they don't have unlockable bootloaders (which is a hard requirement for me), and the underpowered Unisoc/Mediatek chips combined with their god-awful stock ROMs make them way slower than the Galaxy Core LTE I had 10 years ago.