Hi , Thanks for posting in the community about your issue . Signature update is quite a hassle in USG

but it's not a big deal . You can resolve this by step by step troubleshooting:
1. Ping sec.huawei.com from USG
2. Check the ip for sec.huawei.com from PC
3. Check the DNS IP address
4. Check security policy from local to untrust. Make sure the action permit for http and ftp
The step by step procedure is described in huawei official support website . you can follow the official guide to get a vast idea :
https://support.huawei.com/enterprise/en/knowledge/EKB1001215444