Can you show your config, i wonder what's the cpu affinity on this 2 cpu router:
cat /proc/interrupts; grep . /proc/irq/*/smp_affinity_list
on ZBT WG3526 with MT7621A which should be 2 cores but shows 4 in /proc/cpuinfo defaults are that ethernet irqs are handled by all cpus (0-3), but each radio is pinned to a specific cpu:
# cat /proc/interrupts|grep -E "mt7|ethernet"
22: 11376422 0 0 0 MIPS GIC 10 1e100000.ethernet
24: 23 0 0 0 MIPS GIC 11 mt7603e
25: 2 0 98994760 0 MIPS GIC 31 mt76x2e
# grep . /proc/irq/{22,24,25}/smp_affinity_list
/proc/irq/22/smp_affinity_list:0-3
/proc/irq/24/smp_affinity_list:3
/proc/irq/25/smp_affinity_list:2
# grep . /sys/class/net/*/queues/*/rps_cpus
/sys/class/net/br-lan/queues/rx-0/rps_cpus:0
/sys/class/net/eth0.1/queues/rx-0/rps_cpus:0
/sys/class/net/eth0.2/queues/rx-0/rps_cpus:0
/sys/class/net/eth0/queues/rx-0/rps_cpus:e
/sys/class/net/lo/queues/rx-0/rps_cpus:0
/sys/class/net/wlan0/queues/rx-0/rps_cpus:e
/sys/class/net/wlan1/queues/rx-0/rps_cpus:e