Your WNR3500L has BCM53115 switch chip which does support 4K VLAN. That offset VLAN is for switch chip with VLAN 16 limit. Broadcom gigabit switch such as BCM53XXX & some of the newer 100mbit switch chip support 4K VLAN.

That is could be due to incomplete 4K vlan support in some of the codepath in shibby (but I'm sure toastman build had the support for it) or you didn't enable the experimental support for it IIRC. I used tomato toastman for Belkin F7D4301, which had the same chip as your WNR3500L. Back when 4K vlan first added to tomato, not all codepath (K24, K26, K26RT-N etc) had it implemented & I have to manually patch & compile my own. But that should be all fixed (toastman at least) since 2014 & I don't even have to custom compile it.

I can confirm Shibby build don't have the 4K VLAN properly supported for BCM53115. Toastman build works till RT-VLAN 7508.2 & RT-N-VLAN 0508.2. 7509/0509 onwards had Shibby code base merged to it & 4K VLAN for BCM53115 no longer working. You can try to submit bug or contact toastman or shibby to fix it. The code file bcmrobo.c.

So probably time to say goodbye to tomato firmware. I don't recommend tomato for anything other than Broadcom MIPS platform anyway. No point supporting newer hardware (issues with hardware accelerations, wifi power amp, etc) if it is poorly supported.

The 4K VLAN for BCM53115 is not related to your case at all. In fact it is on separate code path. Maybe it's good enough for you, but tomato arm does not meet my requirement for a stable use. Broadcom ARM are mostly newer AC routers (Asus RT-N18U is an exception). If you have Asus Broadcom ARM based router & supported by AsusWRT-Merlin (not for RT-N18U), use that instead. Just because there is build for your device, doesn't mean it is properly supported. This is especially true with OpenWRT & DD-WRT. No point making your network less stable just to have some fancy features.