API request failed on iOS 11.3/11.4 when called through VPN connection

Question

VPN connection created successfully on iOS 10 and also API request executes successfully.

Also, VPN connection created successfully on iOS 11.3 and 11.4 but API request get failed and showing below error:

2018-06-06 15:50:42.366512+0800 myAPP[555:197322] [] nw_path_evaluator_request_nexus_and_stats NECP_CLIENT_ACTION_REQUEST_NEXUS_INSTANCE [65: No route to host]
2018-06-06 15:50:42.366629+0800 myAPP[555:197322] [] nw_endpoint_flow_setup_channel [33 192.168.0.1:8000 in_progress channel-flow (satisfied)] failed to request nexus instance: Error Domain=kNWErrorDomainPOSIX Code=65 "No route to host" UserInfo={NSDescription=No route to host}
2018-06-06 15:50:42.366734+0800 myAPP[555:197322] TIC TCP Conn Failed [18:0x1d0374580]: 1:65 Err(65)
2018-06-06 15:50:42.366808+0800 myAPP[555:197322] Task <2FDCE10C-8CB4-469D-AAE6-E9307A68069D>.<1> HTTP load failed (error code: -1004 [1:65])
2018-06-06 15:50:42.367049+0800 myAPP[555:197506] Task <2FDCE10C-8CB4-469D-AAE6-E9307A68069D>.<1> finished with error - code: -1004


Error-Optional(Error Domain=NSURLErrorDomain Code=-1004 "Could not connect to the server." UserInfo={NSUnderlyingError=0x1d0650f80 
    {Error Domain=kCFErrorDomainCFNetwork Code=-1004 "(null)" UserInfo={_kCFStreamErrorCodeKey=65, _kCFStreamErrorDomainKey=1}}

Thanks