Connecting two Ethernet cables to the router fixes an issue? How is this happening?

Okay, I'm an absolute newbie coming to computer networking. Just started working in the field and theres nobody to provide guidance in my company for such situation, as its a real estate small company.

At my work, I have an Orbi RBR50 router that is getting a connection from the wall Ethernet port directly from the ISP. We have around 40 devices in total including users PCs and phones.

Issue I'm constantly facing is, 2 phones, of different brands (samsung and miui) constantly face connection issues, giving the error "Obtaining IP Address failure" whereas other devices if disconnected, and then reconnected take a good minute before they get connected, most of the type is spent on "Obtaining IP address" but then they usually connect fine. But like i mentioned, the 2 devices are having the connection failure.

I thought maybe, there was some issue the way, the router was assigning the IPs so, I reset the router, but no luck. And there arent any options configured that disable access to the particular devices.

So, then we looked at the port, maybe thats causing the issue, fixed the port, no luck, changed the cable, temporarily its resolved, but it is recurring.

Today i tried something completely bonkers.

I left the original cable plugged which is from the isp. And as we have two Ethernet ports coming in from isp in the wall, connected a secondary Ethernet cable, and put it in the Ethernet-Out port, the one which use to connect the router to our devices. Absurdly enough the phones started connecting completely fine.

I know this is completely stupid and but i dont know how it solved the issue. Does anyone have any idea as to whats actually happening?

Btw, the general troubleshooting such as forgetting network on device, restarting router, resetting router, rebooting affected device etc has been already done but the problems stay. And today suddenly it just works. The wifi works completely fine one day, and the next it is back to being troublesome in those particular devices.

Hopefully someone could shed light on how i can troubleshoot this without contacting the ISP, thats another problem. Dont want to get into that, but its out of the question.