I have a brand new Samsung Galaxy S9+, upgraded from my S7. I never had a problem sending pictures and video by Message+ over WiFi before, in fact it was faster. But whenever in on WiFi (Home or Work) my pictures will not send. They fail every time. I have to turn off WiFi and use 4G signal for them to send. At home, that means I can't send any photos at all because I only have 1x reception there. This is enough to make me return my phone if I cannot find a configuration to fix it!
I'm keeping my S7. It has none of the issues S8 and S9 users are experiencing. If you read the S8 and S9 user reviews on the VZW website, they're weak 4 of 5 stars with consistent complaints about the Bixby button, wifi battery usage, etc. I won't pay nearly $1K for a phone with these kinds of problems.
In my experience, Verizon MMS has always required access to the data network. It is odd that you have to turn off your wifi in order to use it if your data is on. I recommend reaching out by phone to Verizon in order to get technical assistance.
What troubleshooting steps have you attempted since you started having this issue? When did you first start having these issues?
It doesn't matter what device you're using. Verizon has had issues managing MMS since I started using them in 2004. You'll be told to clear caches, delete competing apps, and factory reset your phone. And then you'll notice your issue wasn't fixed. The only method I've found around this is to send the MMS, then restart your phone. Even then you'll likely have to enter the message to see text telling you it failed to send. Tap it again and it usually sends just fine. Maybe in the future when brain to brain communication is invented Verizon will finally put an engineer on this issue, though it will be obsolete. (O_o)
Honestly of all the tech issues I've had with Verizon phones, this is the one that crops up no matter the device, plan, location.