Brand new oven won't heat
There are some good questions here in the comments of your post.

One thing you did not mention was the 125V measurement how that was taken from what to what.

So lets assume you have a 3-Way or a 4-Way hookup, you should have L1, L2, Neutral and if you have a 4th wire it will be the Ground.

Measuring 

L1 to Neutral  125 VAC
L2 to Neutral  125 VAC
L1 to L2       240 VAC
L1 to Ground   125 VAC
L2 to Ground   125 VAC

Neutral to Ground  ~0 to .5 VAC.

Now you do not mention what the installers did - replace wiring or Circuit Breaker or leave the old breaker? 

You should measure your voltage at the Breaker there should be a single or a ganged breaker that is for L1 and L2 - you should measure your voltage on the breaker terminals to see if you have proper voltage right after the breaker (seeing as you did not have it at the oven ???? ). 

1: if you have voltage - turn the oven on and measure the voltage - if the voltage drops you have a bad breaker.

2: if you don't have voltage - you have a bad breaker.

3: If you have voltage at the breaker and while the oven is on as well - verify your wiring connections at the breaker, that they are tight. Then verify the neutral and ground connections are also tight. After this you will need to go to your oven and check the connections at the oven to make sure they are properly connected in a manner appropriate for your ovens connections.

4: If after these steps you still have no voltage - you have a wire break between the service panel and the oven.