Small office setup. could the problem be my router? (image of setup provided)

Hi all,

I'll try and keep this succinct, wish me luck. I'm not a networking savvy guy but am good with computers. We're software developers but we're not good with hardware setups as you'll obviously see.

Here is the office setup:

• Plug from wall socket to modem/router bringing in broadband.
• Attached to modem/router are a HP server, an Apple Time Capsule, a WiiFi extender.
• We connect to the WiFi extender from my Ubuntu machine, some macs, some windows machines, etc. A few of us all with our own dev kits.

Here is an image showing the setup fully (only one machine shown as wifi, but there's more): https://imgur.com/a/vpKKKL7

The problem

We've been having a LOT of dropouts of our broadband. We've had techs come to the office found problems, fixed them etc, but they're still happening. It will drop out for up to 8 minutes at a time and do this as many as 20 times a day. It's insane.

Why I suspect it may be the router:

We've been having issues connecting to our HP Server. Our backups weren't working. For example I tried to connect to Open Media Vault (software that runs our server) and it would be so laggy it wouldn't connect. Things were bad but I thought it was a separate issue to the broadband as the HP Server is a LOCAL connection, it doesn't need internet. I mean, if the internet were totally down, I should still be able to connect to the HP Server as it's a local connection.

BUT, in the middle of an attempt to connect to the server (failing again) the broadband internet sped up and returned to good service. I could browse the net, do speed tests fast, all was good AND I COULD CONNECT TO THE HP Server perfectly.

This got me thinking. If I was having trouble connecting to a local server and the internet was out, then when the internet got better I could connect to the local server again, wouldn't that mean the modem is at fault?

Can anyone weigh in on this before we head out to purchase a new modem? No point in dropping hundreds of $ if we don't have to. Would love some feedback / advice from people who fully understand networking hardware setups (as I truly do not as you can tell).

Thanks so much.