networking - Devices suddenly can't access local IPs on the LAN?

Suddenly, the devices on my LAN can't browse to the IPs or hostnames of other devices on the LAN (only to themselves and to the router at 192.168.1.1).

I have a Win 10 desktop, a Macbook Air, an iPhone 6, and an old iPad, and they are connected to my wifi and are all browsing the internet fine on my Netgear WNDR3700 router with DD-WRT v3.0-r31924 std (05/02/17).

I'm also able to reach the config page of my router at 192.168.1.1 from the devices.

Until a few days ago, they were also able to browse my LAN to show sites I'm developing on my local server (the Win 10 desktop at 192.168.1.120, which is also running Laravel Homestead Vagrant as another server, and everything had been working perfectly til recently).

Now, however, no device is able to connect to LAN IPs other than the router homepage at 192.168.1.1.

I've tried rebooting the devices, rebooting my router and cable modem, restoring my router settings to an earlier version, etc.

Using the "Network Tools" app by KeepSolid on iPhone shows that Trace Route works for normal sites but then fails for LAN sites such as 192.168.1.120.

The "Network Tools" iPhone app also has a DNS lookup utility. When I type in my local hostnames, they point to the proper IP addresses as the "A" record. Other records (AAAA, MX, NS, SOA, SRV, and TXT all say "DNS server returned answer with no data"). I don't really know what this means, and I guess it might all be irrelevant since hostname resolution is useless if my devices can't even visit an IP address directly in a browser.

I have no idea what changed this week... except somehow I noticed that my cable provider changed my WAN IP. But I really doubt that could cause these problems.

I discovered reports of a bug in the DD-WRT firmware on my Netgear WNDR3700 router.

People in that thread suggest that running the following commands in the router would be a suitable workaround, but none of their commands (such as these) has fixed the problem for me:

swconfig dev switch0 set enable_vlan 1
swconfig dev switch0 set apply

Plus, my router had been working, and I didn't make changes to it recently. So it doesn't make sense that this bug would be affecting me.

Plus, I also just tried replacing my Netgear router with an older TP-Link router (running a much earlier version of DD-WRT), and although that router had worked fine before, now even with that one, my devices can only browse the internet but can't ping LAN IPs.

What else can I try to diagnose and fix this problem?