You can't.

LAN is Local Area Network. If you can't connect to said local area network, you can't use it.¹

In other terms: The name of the wifi for both the hoster and player (both macs I assume) needs to be the same (i.e. both connected to same modem)

I'm not sure if you can use the neighbor's wifi as a stand-in (see the footnote below). Worth a shot though. (assuming consent is given)

As @Tas also noted in the comments, connecting the computers directly should work (with an ethernet cable). However if they aren't connected to the internet, things like skins probably won't show up.

^{1: LAN is weird. In my experiences, it usually means the set of computers connected to the same modem. But it can also mean the set of computers connected to the same internet cable/antenna/magic-receiver-thingy. (what the modem connects to) But most likely it means your wifi network in this case.}

If you plug your computer straight into the router with an Ethernet cable and everything works fine then your Internet connection is probably fine it's your WiFi that's messed up.

Assuming you've ruled out Ethernet because you can't run a line, there is another option: Ethernet Over Power aka Powerline aka HomePlug aka IEEE 1901. This uses your house's electrical wiring as a network line. You get two little boxes, one from your computer and one for your router. You plug them into a normal electrical socket, one near your computer and one near your router, then plug a normal Ethernet line into both of them. I use them for a gaming PC which is too far away from the WiFi router for a reliable connection. TP-Link AV500s go for about $40 a pair.

Note: this only works inside your own house. Though I suppose you could run an extension cord to your neighbor's house... you didn't get that idea from me.

Short Answer : You cannot. Long Answer : LAN (local area network) This term means anyone in close range or even connected to your Internet or Router can share files, printers, games, or any other applications. In order to share these types of categories you will need to have Internet Access with someone connected to your router allowing them to share these things.