Did it work before in the past?

There was some issue with 3G network in US. More here https://www.koodomobile.com/en/help/3gshutdownUS

You could try to reset network settings and see if the pixel can connect to LTE and VoLTE can work in the phone?

I went into an AT&T store, and they tried one of their sim cards in my phone, and it worked, so it's not the phone itself. (the guy at AT&T didn't provide any help other than that).

I've tried rebooting my phone, disable and re-enabling the e-sim, and reset the apn's to default.

If so, the issue could be Koodo prepaid. It doesn’t support VoLTE in Canada so I assumed it didn’t support VoLTE in US. 

Btw, did it work before in US?

This site says that roaming includes Alaska…

https://www.koodomobile.com/en/help/long-distance-travel-and-roaming-prepaid

The last time i was in Alaska, I was in Skagway, and I don't think Koodo worked either (it was only a day trip, so I didn't look into it). I have used Koodo in the usa before, but with a different phone a few years ago.

P.S. Thanks for helping, I appreciate it.

Just to update this, I talked with Koodo today, and it appears that with the AT&T 3G shutdown, there is no more coverage in Alaska (as T-mobile does not have towers there).

Correct.  Koodo roams with both AT&T and T-Mobile and both have either finished or in the process of decommissioning their 3G networks.  Koodo prepaid voice calls only work on the 3G network.  Hopefully Koodo gets VoLTE enabled soon.

Mobile data should be available on LTE provided there is actual coverage by one of those two roaming partners.    You mentioned the local AT&T sim worked, so I’m surprised that AT&T didn’t allow you to connect and use mobile data at least.  Did you have roaming enabled for mobile data in your phone?

I would contact Koodo and ask to be credited for the roaming boosters you purchased.