I know this is possible and I see numerous articles but I'm not sure what exactly I need to do -- or what the right terms are so I can search the guides for them.
Right now I use my TP-Link Archer C7 with OpenWRT as my home router. I want to disable the router part and have it act just as a switch and wireless access point. I'm basically going to plug it into another router.
How can I do that? I think I just need to disable NAT and DHCP server -- is that right?
I have a build that will turn your c7-v2 if it is a v2! to a dumb AP would you like to give it a go?
Would I have to reflash or can I do in place upgrade?
I find it easier to configure something "new" from a clean slate, rather than trying to unwind previous configuration.
My suggestion would be to use a clean flash of 18.06.2 just so that you've got a stable starting point. If you're already on 18.06.2, then resetting with firstboot would be my suggestion. tapper's offer would also be a good way to go, based on my opinion of tapper's knowledge and reputation here.
@jeff yes, sorry. I meant to direct that to @tapper. I am on 18.06.2 right now. So if I can't use tapper's build, I will do a system > reset to defaults in LuCI. But will try tapper's build first.
@jeff Thanks for the confidence vote! 
OK the build is for the C7-v2 and I have removed firewall. dnsmasq, opkg, odhcp and I built it with the verry basic LUCI. I will post a list of packages and a link to the build when the build is dun. It will not be to long from now. You should flash and don't save configs. The IP will be 192.168.1.1 but to run it as a dumb AP you should change it to 192.168.1.2 if your mane router is at 192.168.1.1
You can do that under network > lan You can remove the wan and wan6 interfaces.
I know this is an openwrt forum and I have much respect for openwrt, but your best bet, and what I did to convert my outdated router into a dumb ap/switch, flash it with gargoyle fw. Dead simple with gargoyle fw to set dumb ap mode.
I am building a pfSense FW which will be my router so I just need my existing router as switch/AP.
@tapper I await with baited breath. 
@cantenna Since I'm using pfSenses I don't need gargoyle to be my FW.
@cantenna Yes gargoyle is good at being a repeater bridge it is not quite the same. Gargoyle is grate softwair tho.
OK here we go with the build! I have never had any one use a build of mine befor so you are popping my cherry! 
Firmware Version
OpenWrt SNAPSHOT r9814-7f3facfce2 / LuCI Master (git-19.093.68468-d679b93)
Kernel Version
4.14.109
List of packages:
Build:
Pleas remember that because of there not being any dhcp or dnsmasq if you want to connect to this with out a router doing the dhcp you wood need to set your device to a IP in the 192.168.1.xx range.
Btw cantenna It's OK mate to talk about Gargoyle as it is built ontop of OpenWrt and they are friends of OpenWrt.
There is no need for a special build, just to use a router as AP (only) - even less on hardware that isn't below minimum system requirements. https://openwrt.org/docs/guide-user/network/wifi/dumbap is all the configuration you need on a normal build - and you can ignore the more advanced parts of it (the WAN port remapping).
As @slh said, it's simple enough to use the release and change the settings to an AP, and you still have the option to easily revert back to router mode in case need that.
It's only a minute job!
Thanks So I have done a System > Backup / Flash Firmware > Reset to defaults. What order do I do next? Flash your firmware and don't keep settings first? And then set the device IP to 192.168.1.2 and remove the WAN interfaces? Or set the IP and delete the WAN interfaces before flashing?
Hold tight, you're 90% of the way there:
That's basically it...
(You can disable the WAN interface if you want, but with nothing plugged in, it isn't really doing any harm of note)
you can do it before flashing if you like. It will work and then you will not have to giv your PC a static IP. either way will work.
So I tried it but it but got an error message.
The uploaded image file does not contain a supported format. Make sure that you choose the generic image format for your platform.
The router I have is:
| Type | Value |
|---|---|
| Model | TP-Link Archer C7 v2 |
| Architecture | Qualcomm Atheros QCA9558 ver 1 rev 0 |
| Firmware Version | OpenWrt 18.06.2 r7676-cddd7b4c77 / LuCI openwrt-18.06 branch (git-19.020.41695-6f6641d) |
There isn't really any good reason to try to flash the router with a different image, especially if your router is running the recent firmware release. Just do the steps detailed by @jeff. It will take only a minute, and it's easily reversible, should you happen to be a router in the future.
Hi I am sorry to here that, but I don't know why it will not flash. I
flashed it to my c7-v2 before posting. did you use the sysupgrad.bin
file? not the first file I posted.
The .manifest file is just a text file with a list of packages that are
in the build. You need the second file
openwrt-snapshot-r9814-7f3facfce2-ath79-generic-tplink_archer-c7-v2-squashfs-sysupgrade.bin