trying to get tftp upload to work on XS748T

OK, I removed all settings from the switch and started over. I left VLANs strictly alone (the uplink basically only provides one VLAN tag anyway). I'm trying to figure out a reachability/router issue. I think this is the core problem I've been having. Everything I describe is on one VLAN, so we can dismiss that for the moment.  So let's SIMPLIFY what I've got here (maybe I should do another post? :-/ )

There are two networks involved, let's call them A and C. A is our public network space and interfaces out to the internet at large. C is our private network space, where we typically put our switches, CAS, PDU, idrac, IPMI and other network infrastructure.  We have an ubuntu server running 14.04 that is our NAT to the C network.  So it's eth0:0 IP is a A.x.x.3 and its eth0:1 IP address is C.x.x.250. 

The netgear itself gets a static IP address in the C network (C.x.x.102) but it will be providing connectivity to servers that are on either A or C subnets. At present I have one of each connected to the switch (and they're doing fine, actually).

What I am having trouble with is getting the switch to consistently connect with servers (other than the NAT) on the A network.  

Looking at the User Manual (yes, I have been all over this thing), http://www.downloads.netgear.com/files/GDC/XS728T/XS728T_XS748T_UM_EN.pdf to be exact, on page 173, I can see that I need to set the routing as follows:

Network Address: A.x.x.0
Subnet Mask: 255.255.255.0
Next Hop IP Address: C.x.x.102
Preference: 1

(ie the next hop is the IP address of the switch). But the error message is "Gateway cannot be one of the addresses configured on this device."

I do have routing enabled (that's the default) under Routing/IP/IP configuration.

Here's the relevant portion of the manual I'm looking at:

Thoughts? The connection works intermittently (as if various arp tables are briefly in alignment until something times out, but I can't consistently get them to talk.  Then when I did get a connection (see below), I went over to do a traceroute on the switch right

after the traceroute from the A server below. Then immediately the connection was lost. You can see the timestamps before and after on the 'date' commands...it's within a minute :-/  I can consistently ping the NAT and I can consistently ping C subnet servers on any of our switches and I can consistently ping A servers that are connected to THIS switch.  So it's a routing problem. But I can't add the route I want!  I've tried various other things, like listing the A network IP address of the NAT server (that error message response says "Only a directly connected router can be defined as the gateway" -- and that's NAT's C.x.x.250 address).

root@ctrl:~# ping sw-b11
PING sw-b11.private.example.com (C.x.x.102) 56(84) bytes of data.
64 bytes from sw-b11.private.example.com (C.x.x.102): icmp_seq=1 ttl=63 time=1.05 ms
64 bytes from sw-b11.private.example.com (C.x.x.102): icmp_seq=2 ttl=63 time=1.05 ms
64 bytes from sw-b11.private.example.com (C.x.x.102): icmp_seq=3 ttl=63 time=1.09 ms
^C
--- sw-b11.private.example.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2001ms
rtt min/avg/max/mdev = 1.051/1.068/1.099/0.043 ms
root@ctrl:~# arp | grep b11
sw-b11.private.example.com   ether   xx:xx:xx:xx:xx:xx   C                     eth0
root@ctrl:~# date
Tue Oct 30 09:08:00 PDT 2018
root@ctrl:~# traceroute sw-b11
traceroute to sw-b11 (C.x.x.102), 30 hops max, 60 byte packets
 1  sw-b11.private.example.com (C.x.x.102)  4.034 ms  4.127 ms  4.247 ms
root@ctrl:~# ping sw-b11
PING sw-b11.private.example.com (C.x.x.102) 56(84) bytes of data.
From ctrl.public.example.com (A.x.x.8) icmp_seq=1 Destination Host Unreachable
From ctrl.public.example.com (A.x.x.8) icmp_seq=2 Destination Host Unreachable
From ctrl.example.com (A.x.x.8) icmp_seq=3 Destination Host Unreachable
^C
--- sw-b11.private.example.com ping statistics ---
5 packets transmitted, 0 received, +3 errors, 100% packet loss, time 4022ms
pipe 3
root@ctrl:~# arp | grep b11
sw-b11.private.example.com           (incomplete)                              eth0
root@ctrl:~# date
Tue Oct 30 09:09:22 PDT 2018
root@ctrl:~#