@Radiant - Welcome to the forum. Your post is two weeks old and I see no one replied so my post should bump it to the top of the heap.
Have you resolved your network diagram yet?
I'm probably the last person to make suggestions as I'm just a newbie myself, so perhaps the gurus can assist. I'll still toss some questions around.
I wonder why you have a second router attached to your main router rather than using a switch or connecting the computer to the POE switch. You can get an 8-port switch with 4 POE and 4 non-POE or go with all 8 POE in case you want to add more cameras down the road.
The second thing that stands out to me, as I've been schooled here, is that your cameras have a clear path to the outside, they are not isolated on a subnet. That may be a security problem.
Have you bought this gear yet (NVR, cameras) or are you just in the planning stages?
@Radiant - Make sure that you put that second router in "bridge" mode, other wise do like windguy suggested and use a switch and an seperate wifi accesspoint in the garage.
This prevents a "double nat" situation and also prevents 2 DHCP servers in your network.
What network speed do you want to have in the garage?
Cat 6 cable support up to 100m with a speed of 100Mb/sec but it is al depending on interference for the surroundings, more interference slower speed.
Have you checked out the wiki here on this site and espessialy the Cliff Notes?
@Radiant - Since you haven't bought any hardware yet and are just planning, you've passed the first test of this forum, as proctored by the gurus here "Study before buying".
Again, my caveat is that I'm just a newbie, but I would consider an alternative setup to using an NVR, which is running Blue IRIS (BI) on a dedicated computer. That's what I plan to do. I haven't bought any hardware yet either as I'm still studying. There are certainly plenty of forumites using NVR's but also a strong following in the BI camp.
I'd also consider going with a dual NIC setup on that dedicated computer and setting up a subnetwork for your cameras so they are isolated from your main network. Not everyone goes this route but there's a strong following on this forum to using this network topology. It makes sense to me and I plan to setup my network that way. A lot of the other security measures like OpenVPN are still a little abstract for me but I'm coming around to how that all works.
Can you add a second Ethernet run to your garage area so you can isolate that second POE switch and cameras? If so, consider the sketch below as an option.
Hopefully the gurus can share their wisdom and experience to better guide you.