Here lies the darker side of the topic. Fortinet, like most major software vendors, does not make an archive of legacy binaries easily accessible on their public front end. They want users on the latest version for security reasons.
If you must deploy an older version of FortiClient VPN, implement these safety measures to protect your system: forticlient vpn download older versions
The most secure method is to contact your Fortinet service provider, MSP, or internal IT team . They manage the FortiGate and will have legitimate access to the official support portal. Here lies the darker side of the topic
If you successfully performed a install, but you still cannot connect, the issue is likely Cipher Suites or Weak Cryptographic Settings . like most major software vendors