[Google Pixel 2 Questions & Answers] dijit4l:
This was also posted as a reply on the Pixel 2 XL Q&A forum here and I figured other T-Mobile Pixel 2 users would find this useful since this fix works on both the Pixel 2 and Pixel 2 XL.

Situation

My wife was unable to send MMSs on T-Mobile with her Pixel 2. Finally realized it had to do when connected to Wi-Fi, but only when connected to the EAP Wi-Fi at work. Since we work for the same company, I tested with my Pixel 2 XL and it is happening to me too. It works fine at home on our Wi-Fi and on the mobile network, but not when we are on our work's Wi-Fi. I can disable Wi-Fi at work and the MMS sends. Both phones have the same dysfunction. I normally use Google Voice and I never receive MMSs directly to my phone and had not noticed the issue up to this point.

I did a test where I added my Google Voice number and my wife's number in a group message with my phone's number and tested connecting on Wi-Fi, mobile network, and work Wi-Fi and every time we are on the work's Wi-Fi, MMSs fail to send or receive. I know it is happening to only Pixel 2s and no one else because my wife's previous phone, a T-Mobile Galaxy S7, worked fine on any network to send/receive MMSs. I have reset network settings and APN settings back to default, no change. From what research on the issue I could find, people were changing APN settings. Looking at the APN settings, I realized the two T-Mobile MMS listings have the APN listed as "TMUS," whereas the default APN is listed as "fast.t-mobile.com." Since "TMUS" is not a valid domain name, I changed the first T-Mobile MMS APN to fast.t-mobile.com and MMS started working on the work's Wi-Fi. I reset my APN settings back to default and it stopped working on my work's Wi-Fi again (as expected). I thought that I should leave the default APNs alone, just in case, and decided to create a third T-Mobile MMS APN with fast.t-mobile.com as the APN. This third T-Mobile MMS APN somehow allows MMSs to work on the company's Wi-Fi.

Fix

This fix seems to be the least amount of work and is very easily undone.

Go to Settings > Network & Internet > Mobile network > Access Point Names.

Tap on the 3 dot menu at the upper-right and select "Reset to default."

Create a new APN with the following settings:

Name: T-Mobile MMS

APN: fast.t-mobile.com

Proxy, Port, Username, Password, and Server: Not Set

MMSC: http://mms.msg.eng.t-mobile.com/mms/wapenc (When typing this in, make sure auto correct doesn't change anything since URLs are case sensitive!)

MMS Proxy and MMS Port: Not Set

MCC: 310 (Should already be set)

MNC: 260 (Should already be set)

APN Type: mms

APN Protocol: IPv6

APN Roaming Protocol: IPv4

Bearer: Unspecified

MVNO Type: None

MVNO Value: Not Set

Tap on the 3 dot menu and select "Save."

Reboot your phone for good measure (may not be necessary) and open Android Messages.

Send a MMS or group MMS on and off the Wi-Fi network you are having issues on to see if it sends. If you have several queued up messages that haven't sent, you may need to tap to resend them and they may become out of order on the recipient's end.

If you ever wanted to undo these changes, you can either reset the APN settings or remove the fast.t-mobile.com one.

Conclusions

The main thing to note about this is that it is the same as the other T-Mobile MMS APNs, but has TMUS replaced with fast.t-mobile.com. If you look at the actual default fast.t-mobile.com APN, you'll notice that "mms" isn't listed among the others in APN Type. I feel, if I could edit that default fast.t-mobile.com APN, the problem would be fixed by adding ",mms" at the end. I assume one could duplicate everything in the default one and add it that way, but I wanted to provide the simplest, non-invasive fix possible that would not result in any future or unforeseen problems. When the phone/Google Messages/text message app consults the APNs while on an enterprise Wi-Fi connection, I assume it goes through each one to attempt to find one with MMS in the APN type and then attempts to contact the APN via its domain name, but fails since TMUS is not valid. I am not sure why it doesn't go ahead, verifies a working data connection, and then sends the MMS using http://mms.msg.eng.t-mobile.com/mms/wapenc as it seems to do fine on other Wi-Fi networks. The phones are like, "Whelp, I'm on an enterprise Wi-Fi connection and I need me an MMS APN with a FQDN or I aint workin'!" I do not know what makes my company's Wi-Fi so different from my home Wi-Fi aside from it using EAP authentication. This wasn't a problem on my wife's previous phone (T-Mo Galaxy S7). I had the 2016 Pixel for a while, but I use Google Voice and would not normally notice the MMS issues.

If anyone has a good theory as to wth is going on, please contribute! I think its some sort of wonky Oreo issue. I hope this helps anyone who was as frustrated as I was!