According to this document you can patch the boot ROM to preset various configuration values, and one of them is the TFTP server address.
It looks like you still require a DHCP server in order to supply an IP address and netmask to the Pi itself, as without this the Pi won't have the necessary information to contact your preset TFTP server.
You'll need to follow the instructions at that link as they are a bit long to repost here, but the gist of it is:
cp /lib/firmware/raspberrypi/bootloader/beta/pieeprom-2019-09-23.bin pieeprom.bin
rpi-eeprom-config pieeprom.bin > bootconf.txt
# Edit bootconf.txt and set TFTP_IP=126.96.36.199
rpi-eeprom-config --out pieeprom-netboot.bin --config bootconf.txt pieeprom.bin
sudo rpi-eeprom-update -d -f ./pieeprom-netboot.bin # Flash updated boot ROM
Basically you extract the boot ROM config into a text file, edit it, then merge it back to the ROM file and reflash it.