MM fiber to SM back to MM?

We have just moved our office into a new building, and I (company network admin) did not handle the ISP communications for the new site, or the inside wiring (my boss did that, and I answered his questions along the way.) So we moved into the new space, and it came time to turn up the new circuit, and I found that unlike our prior location where the ISP terminated their circuit in our data center, all the ISP circuits terminate in a office park MDF, and then the building’s management co has an authorized inside wiring contractor extend the fiber into our suite (only that wiring contractor and the ISP techs are allowed into the MDF - no tenants.) So we had told our ISP to handoff on 1G MM fiber (LC connector), but it turns out that the inside wiring contractor ran single-mode fiber from the MDF to our suite, which I only found out when we went to turn up the circuit, even though my boss had also told them we needed MM. (When he called them to get them to change it, they told him that’s the only way they do runs.)

So now we have an ISP handoff on MM fiber, and our router (Cisco ISR4431) currently has a 1G MM SX SFP... What’s the best way to deal with this situation without having the ISP have to make a change? The circuit turnup was supposed to happen yesterday, and now is rescheduled with the ISP for Thurs, which is delaying the office opening...

Thanks!