2 years passed, I found the solution!!! it was simple! WAN3 (that must not be included into Load Balance) MUST BE ADDED TO LINK BACKUP with "Failover(Enable backup link when all primary WANs fail)."...

Hello @GRL ! Thank you for advice! I've set all my wans to mtu=1400 VPN traffic gone slover but does not fixed iphone upload via l2tp... Please pay attention that download through the same iphone's...

Hello @Hank21 Thank you for fast reply! I have only 2 clients via L2TP VPN. the first based at remote ER6120 L2tp client. And it works fine! The second - is my iPhone X configured it's own ios l2tp...

P.S. also iPhone Disconnecting while uploading 5-10Gb video to WhatsApp through L2TP VPN. All upload operations that more than ~5Gb causing L2TP disconnecting from iPhone..... ((

Hi everybody! I found that iPhones have unstable L2TP VPN to er8411 via LTE network. it disconnecting every time at Speedtest while Upload test. I've tried encrypted and unencrypted servers sets....

Hello @GRL there is no issues from other router types at this IP. only Archer C6/C64 Solution is to set IP as static. Probably its C6/C64 firmware problem.... if anyone have Archer C6 v4.0 fw: 1.13.6...