Use another DHCP server.
Here again, this situation is rather straightforward since that case in handled by the DHCP RFC.
Enable the proxydhcp checkbox and TPS will act a proxy dhcp i.e will not interfere with your standard DHCP process.
It will then only handle the dhcp options dealing with the network boot part.
It will not provide dhcp options such as the leased IP, gateway, etc, and will leave this part to your DHCP server.
A quick analysis of TPS log file.
12:39:45 DHCPd:DISCOVER received, MAC:00-0C-29-B5-50-CC, XID:2AB550CC
#send offer 0.0.0.0 to manifest itself as proxydhcp
12:39:45 DHCPd:OFFER sent, IP:0.0.0.0, XID:2AB550CC
12:39:47 DHCPd:DISCOVER received, MAC:00-0C-29-B5-50-CC, XID:2BB550CC
#again
12:39:47 DHCPd:OFFER sent, IP:0.0.0.0, XID:2BB550CC
12:39:51 DHCPd:DISCOVER received, MAC:00-0C-29-B5-50-CC, XID:2CB550CC
#again
12:39:51 DHCPd:OFFER sent, IP:0.0.0.0, XID:2CB550CC
#discard request on udp:67 i.e dhcp
12:39:59 DHCPd:REQUEST discarded, MAC:00-0C-29-B5-50-CC, XID:2CB550CC
#accept request on udp:4011 i.e proxydhcp
12:39:59 PDHCPd:REQUEST received, MAC:00-0C-29-B5-50-CC, IP:192.168.1.155, XID:2CB550CC
#sent ack on udp:4011
12:39:59 PDHCPd:DHCP_ACK sent, IP:192.168.1.155:68, xid:2CB550CC
#coming tftp read
12:39:59 TFTPd:DoReadFile:ipxe.pxe B:1456 T:0
#coming discover from ipxe
12:40:06 DHCPd:DISCOVER received, MAC:00-0C-29-B5-50-CC, XID:618C202F
12:40:06 DHCPd:iPXE user-class detected
#send offer 0.0.0.0 to manifest itself as proxydhcp
12:40:06 DHCPd:OFFER sent, IP:0.0.0.0, XID:618C202F
#discard request on udp:67 i.e dhcp
12:40:06 DHCPd:REQUEST discarded, MAC:00-0C-29-B5-50-CC, XID:618C202F
#it seems iPXE does not wait for a proxydhcp ack to validate the boot filename
12:40:06 TFTPd:DoReadFile:menu.ipxe B:1432 T:16123