Your current firmware is rather ancient, which is almost assuredly the issue.
If you have access to the support portal, have a look at the Customer Support Bulletin issue # CSB-200106-1:
Starting on 1st May 2020, FortiToken Mobile registration servers will enforce TLS v1.2 protocol version. [...] After the 1st May 2020 all FortiGates requiring the FortiToken Mobile service must be running FortiOS software version v5.6 or later in order to support TLSv1.2 for FortiToken Mobile registration. ( https://support.fortinet.com/Information/Bulletin.aspx )
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