I don't think ViaBTC restricts miners from using proxies.
Have you tried to connect your miner without the proxy? If it's connecting properly, meaning the proxy might be the issue, you need to change it to maybe HTTP proxies instead of using socks5 and proxies that encrypt data sent to the pool server.
Would you mind to share the unit model of your miner?
And the type of proxy you use(HTTP, Socks4, Socks5)?
I'm renting hashpower through MRR for SHA256 Asicboost mining (1.12Ph/s advertised). The miner is located in Russia and I'm testing a custom Stratum proxy to handle pool connections. Here are the relevant details from MRR:
RIG: 328097
Type: SHA256 Asicboost
Location: Russia
Current hashrate: ~28 TH/s
The proxy is a direct Stratum protocol implementation, not HTTP/SOCKS.