In case you encounter the same problem, here is my solution.
After adding the pppd option debug
config 'interface' 'wan'
...
option 'pppd_options' 'debug'
to the interface configuration, I realized that the IPCP negotiation failed:
Jul 11 17:36:01 fon4 daemon.info pppd[1057]: CHAP authentication succeeded
Jul 11 17:36:01 fon4 daemon.notice pppd[1057]: CHAP authentication succeeded
Jul 11 17:36:01 fon4 daemon.debug pppd[1057]: sent [IPCP ConfReq id=0x1 <addr 10.0.1.4> <ms-dns1 0.0.0.0> <ms-dns3 0.0.0.0>]
Jul 11 17:36:02 fon4 daemon.info pppd[1057]: Hangup (SIGHUP)
Jul 11 17:36:02 fon4 daemon.notice pppd[1057]: Modem hangup
Jul 11 17:36:02 fon4 daemon.notice pppd[1057]: Connection terminated.
Changing the local address only made for a longer negotiation
Jul 12 01:03:08 fon4 daemon.info pppd[1090]: CHAP authentication succeeded
Jul 12 01:03:08 fon4 daemon.notice pppd[1090]: CHAP authentication succeeded
Jul 12 01:03:08 fon4 daemon.debug pppd[1090]: sent [IPCP ConfReq id=0x1 <addr 192.168.10.1> <ms-dns1 0.0.0.0> <ms-dns3 0.0.0.0>]
Jul 12 01:03:09 fon4 daemon.debug pppd[1090]: sent [LCP EchoReq id=0x1 magic=0x86ca2654]
Jul 12 01:03:09 fon4 daemon.debug pppd[1090]: rcvd [LCP EchoRep id=0x1 magic=0xf66dc8 86 ca 26 54]
Jul 12 01:03:09 fon4 daemon.debug pppd[1090]: rcvd [IPCP ConfNak id=0x1 <ms-dns1 10.11.12.13> <ms-dns3 10.11.12.14> <ms-wins 10.11.12.13> <ms-wins 10.11.12.14>]
Jul 12 01:03:09 fon4 daemon.debug pppd[1090]: sent [IPCP ConfReq id=0x2 <addr 192.168.10.1> <ms-dns1 10.11.12.13> <ms-dns3 10.11.12.14>]
Jul 12 01:03:10 fon4 daemon.debug pppd[1090]: sent [LCP EchoReq id=0x2 magic=0x86ca2654]
Jul 12 01:03:10 fon4 daemon.debug pppd[1090]: rcvd [LCP EchoRep id=0x2 magic=0xf66dc8 86 ca 26 54]
Jul 12 01:03:10 fon4 daemon.debug pppd[1090]: rcvd [IPCP ConfNak id=0x2 <ms-dns1 10.11.12.13> <ms-dns3 10.11.12.14> <ms-wins 10.11.12.13> <ms-wins 10.11.12.14>]
Jul 12 01:03:10 fon4 daemon.debug pppd[1090]: sent [IPCP ConfReq id=0x3 <addr 192.168.10.1> <ms-dns1 10.11.12.13> <ms-dns3 10.11.12.14>]
Jul 12 01:03:10 fon4 daemon.info pppd[1090]: Hangup (SIGHUP)
Jul 12 01:03:10 fon4 daemon.notice pppd[1090]: Modem hangup
Jul 12 01:03:10 fon4 daemon.notice pppd[1090]: Connection terminated.
So I reverted to my old local address and added the pppd option noipdefault
config 'interface' 'wan'
...
option 'pppd_options' 'debug noipdefault'
Now the connection can be properly established:
Jul 12 13:14:12 fon4 daemon.info pppd[2802]: CHAP authentication succeeded
Jul 12 13:14:12 fon4 daemon.notice pppd[2802]: CHAP authentication succeeded
Jul 12 13:14:12 fon4 daemon.debug pppd[2802]: sent [IPCP ConfReq id=0x1 <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns3 0.0.0.0>]
Jul 12 13:14:13 fon4 daemon.debug pppd[2802]: sent [LCP EchoReq id=0x1 magic=0x7e357d3c]
Jul 12 13:14:13 fon4 daemon.debug pppd[2802]: rcvd [LCP EchoRep id=0x1 magic=0xfeb5e2 7e 35 7d 3c]
Jul 12 13:14:13 fon4 daemon.debug pppd[2802]: rcvd [IPCP ConfNak id=0x1 <ms-dns1 10.11.12.13> <ms-dns3 10.11.12.14> <ms-wins 10.11.12.13> <ms-wins 10.11.12.14>]
Jul 12 13:14:13 fon4 daemon.debug pppd[2802]: sent [IPCP ConfReq id=0x2 <addr 0.0.0.0> <ms-dns1 10.11.12.13> <ms-dns3 10.11.12.14>]
Jul 12 13:14:14 fon4 daemon.debug pppd[2802]: rcvd [IPCP ConfReq id=0x0]
Jul 12 13:14:14 fon4 daemon.debug pppd[2802]: sent [IPCP ConfNak id=0x0 <addr 0.0.0.0>]
Jul 12 13:14:14 fon4 daemon.debug pppd[2802]: rcvd [IPCP ConfNak id=0x2 <addr 10.230.6.29> <ms-dns1 139.7.30.125> <ms-dns3 139.7.30.126>]
Jul 12 13:14:14 fon4 daemon.debug pppd[2802]: sent [IPCP ConfReq id=0x3 <addr 10.230.6.29> <ms-dns1 139.7.30.125> <ms-dns3 139.7.30.126>]
Jul 12 13:14:14 fon4 daemon.debug pppd[2802]: rcvd [IPCP ConfReq id=0x1]
Jul 12 13:14:14 fon4 daemon.debug pppd[2802]: sent [IPCP ConfAck id=0x1]
Jul 12 13:14:14 fon4 daemon.debug pppd[2802]: rcvd [IPCP ConfAck id=0x3 <addr 10.230.6.29> <ms-dns1 139.7.30.125> <ms-dns3 139.7.30.126>]
Jul 12 13:14:14 fon4 daemon.warn pppd[2802]: Could not determine remote IP address: defaulting to 10.64.64.64
Jul 12 13:14:14 fon4 daemon.notice pppd[2802]: local IP address 10.230.6.29
Jul 12 13:14:14 fon4 daemon.notice pppd[2802]: remote IP address 10.64.64.64
Jul 12 13:14:14 fon4 daemon.notice pppd[2802]: primary DNS address 139.7.30.125
Jul 12 13:14:14 fon4 daemon.notice pppd[2802]: secondary DNS address 139.7.30.126
Jul 12 13:14:14 fon4 daemon.debug pppd[2802]: Script /etc/ppp/ip-up started (pid 2821)
Jul 12 13:14:17 fon4 user.notice root: adding wan (3g-wan) to firewall zone wan
Jul 12 13:14:19 fon4 user.notice rdate: No usable time server found
Jul 12 13:14:19 fon4 daemon.debug pppd[2802]: Script /etc/ppp/ip-up finished (pid 2821), status = 0x1
Jul 12 13:14:22 fon4 daemon.info dnsmasq[2467]: reading /tmp/resolv.conf.auto
Jul 12 13:14:22 fon4 daemon.info dnsmasq[2467]: using nameserver 139.7.30.126#53
Jul 12 13:14:22 fon4 daemon.info dnsmasq[2467]: using nameserver 139.7.30.125#53
Jul 12 13:14:22 fon4 daemon.info dnsmasq[2467]: using local addresses only for domain lan