Random disconnect of OpenVPN connection on AC86U

[vulnerable]

I just configured a new AC86U for OpenVPN connections. I am able to connect successfully. However I experienced random disconnect, from some seconds to minutes. I found the following logs on the router.

Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 TLS: soft reset sec=0 bytes=3715859/-1 pkts=10564/0
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 peer info: IV_VER=2.5_master
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 peer info: IV_PLAT=android
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 peer info: IV_PROTO=2
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 peer info: IV_NCP=2
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 peer info: IV_LZ4=1
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 peer info: IV_LZ4v2=1
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 peer info: IV_LZO=1
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 peer info: IV_COMP_STUB=1
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 peer info: IV_COMP_STUBv2=1
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 peer info: IV_TCPNL=1
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 peer info: IV_GUI_VER=de.blinkt.openvpn_0.6.73
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 PLUGIN_CALL: POST /usr/lib/openvpn-plugin-auth-pam.so/PLUGIN_AUTH_USER_PASS_VERIFY status=0
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 TLS: Username/Password authentication succeeded for username 'username' [CN SET]
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 WARNING: 'link-mtu' is used inconsistently, local='link-mtu 1556', remote='link-mtu 1572'
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 WARNING: 'cipher' is used inconsistently, local='cipher BF-CBC', remote='cipher AES-256-CBC'
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 WARNING: 'keysize' is used inconsistently, local='keysize 128', remote='keysize 256'
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 Outgoing Data Channel: Cipher 'AES-256-CBC' initialized with 256 bit key
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 Outgoing Data Channel: Using 256 bit message hash 'SHA256' for HMAC authentication
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 Incoming Data Channel: Cipher 'AES-256-CBC' initialized with 256 bit key
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 Incoming Data Channel: Using 256 bit message hash 'SHA256' for HMAC authentication
Nov 22 16:18:43 openvpn[1885]: username/x.x.x.x:37342 Control Channel: TLSv1.2, cipher TLSv1/SSLv3 ECDHE-RSA-AES256-GCM-SHA384

For the WARNING messages, I did not configure link-mtu and key size ( I can't even find that it is configurable on the router), and the cipher has already been configured as AES-256-CBC, not BF-CBC. Do these messages relate to the random disconnect? How can I get rid of these messages?
I am using the latest version 382.1
Thanks a lot in advance.