Probably obvious, but I'm stumped (one apartment has no internet)

tl;dr - I run a small, simple network for my HOA. a pfSense box serves 34 units in 7 buildings over cat5e and unmanaged switches. After upgrading one building's unmanaged switch everyone one was fine except one unit. That person still has no connectivity. In pfSense it appears his router is requesting a 192.168.x.x IP, which is not valid on our network. Testing with my old Netgear R7000, pfSense shows that it has a valid IP, but the router itself says it has a 192.168.x.x address and has no connectivity. I've done many, many factory resets of both. pfSense is offering valid IPs but the routers won't pick them up.

Long Version:

I've spent about 5 hours today trying to troubleshoot this and am stumped. I thought I would post here and see if any of you good people have any insight. As usual, it's probably something simple that I'm overlooking.

I run a small network for my HOA (34 units/homes). Here's our network:

1. Cable modem with a Comcast Business line
2. pfSense box
3. Netgear smart managed switch
4. Cat5e out to unmanaged switches in 5 buildings
5. Cat5e from unmanaged switches to units

Yesterday we replaced the unmanaged switch in one building, upgrading from a 10/100 Sixnet to a Gigabit TrendNET switch. We've already done this upgrade in 4 other buildings and it's gone fine. After this 15-minute upgrade one unit out of the 7 in that building said they had no internet access. I've spent about 5 hours troubleshooting today and am still stumped. What I've tried:

• Factory reset his router (5-month old TP-Link Archer A7) many, many times)
• Re-terminated both ends of the cat5e that serves this unit and verified it with a cable tester
• Swapped this unit's cable into 3 other ports on the TrendNet. His unit never got access, but the other swapped units kept their access, regardless of which port was used.
• Connected my laptop to the cable coming into his unit - it got connectivity
• Connected his OLD laptop to the cable coming into his unit - no connectivity
• Lent him my old Netgear R7000 to try - no connectivity (despite many, many factory resets)
• Reinstalled the previous 10/100 switch at his request - no change

In pfSense I can see his router is requesting a 192.168.x.x address which isn't valid on our network, so it sends a NAK. pfSense is offering a valid IP, but the router never takes it. In pfSense my R7000 appears to have a valid IP, but when I look on the router it says it has a 192.168.x.x address and it has no connectivity.

It seems like the problem is with the two routers but I wonder if I'm missing something

• Why would installing a new unmanaged switch cause his router to stop pulling a DHCP lease?
• I had one other resident in that building say that their Linksys mesh setup also stopped working right after the switch upgrade, but they've had a lot of problems with it and they were able to connect their old router and it had connectivity.
• The issue with my R7000 is probably just coincidence, since it's been sitting on a shelf for the last month, but it worked fine before.
• While I know managed switches would be great, they simply aren't in the budget, and everything is working great for all other units in the community.

Thanks for anyone who's made it this far! I'll take any thoughts or tips you've got.