I've been having an issue as well recently I got 3 little hapMinis, they don't have enough space to update so netinstall was needed.
The first one went fine but the next 2 gave trouble...
2nd one kept error "FAILED TO REPLY".. then after multiple commands and setting a different ip on the bridge I was able to do it. Kept having to go into /system routerboard settings set boot-device=try-ether
Now on the 3rd one it didnt want to at all kept sending same "FAILED TO REPLY" message, I tried changing ports ips even admin-mac on the bridge interface to match the first port ether1 which was WAN by default and recofigured to join bridge with the rest of the ports.
I ended up backing up the config and restarting the device then resetting it, upon reset added interface to bridge (ether1) and set new up 10.0.5.3/24 then the boot-device=try-ether and rebooted while netinstall running see below the results:
root ~ Downloads netinstall-6.48.4 ./netinstall -a 10.0.5.3 ../routeros-smips-6.48.4.npk
Using server IP: 10.0.5.4
Starting PXE server
Waiting for RouterBOARD...
PXE client: B8:69:F4:3C:E2:A1
PXE client: B8:69:F4:3C:E2:A1
PXE client: B8:69:F4:3C:E2:A1
PXE client: B8:69:F4:3C:E2:A1
PXE client: B8:69:F4:3C:E2:A1
PXE client: B8:69:F4:3C:E2:A1
PXE client: B8:69:F4:3C:E2:A1
PXE client: B8:69:F4:3C:E2:A1
PXE client: B8:69:F4:3C:E2:A1
Sending image: mips
Discovered RouterBOARD...
Key was rejected
root ~ Downloads netinstall-6.48.4 ./netinstall -a 10.0.5.3 ../routeros-smips-6.48.4.npk
Using server IP: 10.0.5.4
Starting PXE server
Waiting for RouterBOARD...
bind tftp general failed: Address already in use
Discovered RouterBOARD...
Formatting...
Sending package routeros-smips-6.48.4.npk ...
Ready for reboot...
Sent reboot command
root ~ Downloads netinstall-6.48.4 ps aux | grep net
root 2235599 0.0 0.0 36124 3412 pts/8 S 19:23 0:00 ./netinstall -a 10.0.5.3 ../routeros-smips-6.48.4.npk
You can see the error on the key, program exits, you have to start it again yet another instance is running in background and bound to tftp port.. the second instance sends the package and installs it.
On boot its the new OS:
[iaskakho@ap1] > /system routerboard print
routerboard: yes
board-name: hAP mini
model: RouterBOARD 931-2nD r2
serial-number: 94A2096251B5
firmware-type: qca9531L
factory-firmware: 6.42.5
current-firmware: 6.42.5
upgrade-firmware: 6.48.4
[iaskakho@ap1] > /system resource print
uptime: 10m5s
version: 6.48.4 (stable)
build-time: Aug/18/2021 06:43:27
factory-software: 6.42.2
free-memory: 7.8MiB
total-memory: 32.0MiB
cpu: MIPS 24Kc V7.4
cpu-count: 1
cpu-frequency: 650MHz
cpu-load: 3%
free-hdd-space: 7.8MiB
total-hdd-space: 16.0MiB
write-sect-since-reboot: 385
write-sect-total: 2264
bad-blocks: 0%
architecture-name: smips
board-name: hAP mini
platform: MikroTik