On current version I do not see Legacy or System options in Settings. I have been back and forth with Verizon and with Textra about this and unfortunately it looks like (at least in my case) it is Android that is causing the issue. I can send/receive MMS in the stock Messenger just fine but Textra will fail (Timeout: Can't get MMS) or send the message several hours later whereas the stock app goes through instantly.
I have tried a couple of other third-party text apps and the exact same issue, MMS will not send at all or is delayed for hours. I've tried disabling everything I can possibly disable on the stock app to prevent the stock app from "hijacking" the MMS incoming/outgoing, I've disabled all the power management, cleared cache, uninstall/reinstall Textra, resync, everything.
Verizon had me dial *920, the rep said that they had seen cases where "call forward on busy" (which *920 turns off) would prevent MMS from going through, and after I turned it off I was able to send a pic to myself and got it immediately, but the next morning I was back to not being able to send/receive MMS via Textra. I tried *920 a couple of times again and it didn't help.
This all started about 2 weeks ago. So, I can only assume at this point that there was something in the last Android security update that broke this for 3rd party apps.