I had the same problem with a TL-R600VPN. TP-Link looked at it over several weeks and replied as follows​:

(lightly edited for clarity) 

You provided a table of IP and MAC addresses of all your network devices. From the table, I found the total number of devices added up to more than 60. Our router R600VPN can only load 50 clients at most.  In order to maintain the best operation, we suggest the number of clients should be kept at around 30~40. We suspect that the frequent death of the DHCP Server may be related to the load of the router. Please reduce the load and keep the clients around 30-40.

The capacity of the router depends on the hardware performance of the router and nothing to do with DHCP server specifically. If the number of clients is beyond the capacity that the router can carry, then it may crash and stop working. We sincerely suggest that you can reduce the number of clients appropriately.

We have three models VPN router. You can check the information by accessing our official web.
https://www.tp-link.com/en/business-networking/vpn-router/  (*for me this only accessed information on one of the routers)

TL-ER6120 V3 can carry 300 clients at most.
TL-ER6020 V2 can carry 100 clients at most.
TL-R600VPN v4 can carry 50 clients at most. (*note however that previously in the text it is suggested to limit the number of devices to 70% of capacity)

 

 

MichaelAuva wrote

Model :

Hardware Version :

Firmware Version :

ISP :

The internal DHCP-server stops working after some time

Latest firmware TL-ER6120_V1_140113
Tried changing lease time : no sollution

@MichaelAuva