Switched Comcast router and now sporadic internet issues with specific sites - HELP!

OK Reddit networking guru's I've had a day, and need some help!

We have a small (16 person) organization that I am the IT person for. We've been using Comcast for years and just renewed our contract which ended up being cheaper (saved $20/mo) if we upgraded our service to their new Business Security Edge product with a Cradlepoint LTE backup solution that it will fail over to if Comcast goes down.

So Comcast installer comes on site to do install, gets everything installed, "ports over" (or whatever you call it) our static IP address we pay for and the associated settings from old router to new, we test network connectivity, speeds, ensure VPN still works from a hotspot connection as well as from a few remote workers and we are good to go. They go on their way and I start working on my other tasks for the day.

About an hour later I start getting reports from the handful of people working back in the office that they are having strange connectivity issues where they can't reach certain sites.

Super long story / troubleshooting short, here is where we stand:

• When connected to our office network, I can reach most of the internet no issue. However, there are specific sites (such as msn.com, cnn.com, microsoft.com, and our organizations website (which is hosted elsewhere/offsite) that I cannot reach via Chrome, Edge, but can reach via FireFox.
• Of the handful of users in the office my laptop and one other test machine I pulled out can replicate successfully being able to connect to our organizations website via FireFox, but again no other browsers.
• All other staff in the office cannot reach any of those sites, regardless of what browser they are using.
• We have verified that if you are on a hotspot, outside the office etc all sites/browsers work fine.
• If you are outside the office but connected to VPN issues arise again (which of course makes sense).
• I started a Tier 1 ticket with Comcast and they couldn't figure anything out, escalated to Tier 2 but won't be able to get back to me until tomorrow.

So what in the world is going on here? In my mind it HAS to be related to some setting on their new router right? We made zero changes to our firewall, which worked fine with the old Comcast router. The ONLY thing I discovered when I first started trying to troubleshoot this on my own, was that the new router was configured to hand out DHCP, which I ultimately disabled, and rebooted the router, and the firewall, no change in issue.

Our configuration for what its worth is:

Comcast modem with static external IP address > single ethernet to SonicWall NSA2600 Firewall that is configured to hand out DHCP for our two subnets (one a BYOD, the other our VOIP phones) > network switches > physical server with three NIC's set to static IP's.

The server is running Win Server 2012 R2 (side note we'll be upgrading to Azure hopefully next year) and it is handling DHCP for our internal network devices (laptops, photocopiers, printers etc).

Thanks in advance for any insight, pointers, etc. I hope to hear back from Comcast tomorrow and keep working on this, but it bugging me not being able to figure this out, and it seems so weird and sporadic. I certainly am not a networking expert, frankly know enough to get in trouble haha.