Cannot connect to 4G on Mifi

I am using a happy prepaid sim on a tp-link M7450. I have configured it to connect to 4G only and it has been working all along for 6months. Suddenly last week, it shows no service. If I change the setting to 4G preferred, it can connect to 3G and will not switch to 4G at all. I'm stucked on 3G. 

I tried putting the sim into an iPhone and I'm able to get 4G signal tho. 

I am using APN shppd with no username and password (same setting as iPhone)

Can I check why the mifi device cannot connect to 4G? I have not change any setting on the mifi nor did any firmware upgrade. Did starhub change any policy? Anyone facing the same issue on a mifi device?