I have a Canon 70D and the Neewer NW680/TT680 flash. I would like to setup an umbrella stand and use the flash externally. The flash can enter Slave mode 1 and 2 if you hold the M button but I was not able to get it to work with my camera unless the flash is mounted onto the camera. Is this an optical slave mode only? Would getting a trigger like the NEEWER 2.4G Remote Trigger let me remotely use the flash I have?
TL;DR: My advice: return the NW680/TT680, and increase the budget to get a Godox TT685 II-C instead.
Looking over an Amazon listing for the NW680/TT680 for Canon, it appears to be primitive with no built-in radio, and only S1/S2 "dumb" optical slave modes. It does not communicate Canon's "smart" wireless eTTL, where your 70D's pop-up flash could be used as a master unit to remote control it.
In S1/S2, a simple optical sensor will fire the flash when it senses another flash burst. And that's it. You'd have to use the Neewer in M mode, at or below sync speed (1/250s on a 70D) and any settings changes on the flash need to be made directly on the speedlight itself. You'd use S1 if the 70D's pop-up is in M (fires on the first burst sensed), and S2 if the 70D's pop-up is in TTL (skipping over the TTL metering preburst).
Would getting a trigger like the NEEWER 2.4G Remote Trigger let me remotely use the flash I have?
No. Only a few flashes are actually in the Neewer Q system and the NW680 isn't one of them, and Neewer also has probably half a dozen other 2.4GHz radio transmitters, since they rebrand stuff from half a dozen different Chinese companies.
IOW, you kinda got what you paid for. There are reasons to avoid $60 and cheaper flashes. To me, a flash can be more transformative to your photography than a new lens. Budgeting accordingly isn't stupid.
My recommendation [at this time] would be to try and find a Godox TT685-C. While it's mostly discontinued in favor of the TT685 II-C, it can do the "smart" optical communication the 70D's pop-up does. It also has built-in radio. And it'll do TTL and HSS both on and off the camera hotshoe if you pair it with an XPro-C, X2T-C, or XPro II-C Godox transmitter on your camera's hotshoe. And you can use the 70D's flash menus to remote control everything.
The Godox TT685 II-C does not have the "smart" optical capability, but still has the built-in radio triggering.
Radio is often preferred over optical for shooting outdoors in the sunlight where optical signalling can be overpowered by the sunlight and range and reliability are reduced and line-of-sight requirements get more strict without bounce surfaces around. Optical is great in studio conditions; not so great for on location work.
One last note. If you've never used a flash at all before, I'd recommend you start not with an off-camera Strobist setup, but learning on-camera bounce flash. While you have less control, all you need to get started is a speedlight, and you can learn the basics of flash exposure, flash/ambient balance, and controlling the intensity, direction, quality, and (with gels) color of your light. It's much fast and more convenient than off-camera and easier to grasp all the basics. After you hit the limits of bounce flash, then pursue off-camera and things will go a lot faster and easier on the learning curve. Just my 2¢.
The Newer NW660 does incorporate a basic optical Slave Flash system. In S1 or S2 mode it will flash whenever it sees a bright light from another flash.
You can use it, as is, with your 70D, if you use the pop-up flash on your camera to trigger the Newer Flash. Use S1 if you set your camera flash to Manual, and use S2 if you set the camera flash to ETTL.
When using S1 or S2 there would be no need to set the camera flash to wireless. The extra coded light impulses usesd in Canon’s proprietary optical wireless flash system would probably cause problems and not work with the simple optical slave flash system on the Newer flash.