Huawei B660 3G - Can 3G Signal Strength Weaken With More Concurrent Users

I'm using a Huawei B660 3G router at my house, with with a service provider called 8ta (in South Africa). I've been struggling for months to try and get them to troubleshoot and fix my connection, which was working fine for about a year.

I'm now daily struggling with download speeds of as little as 0.1Mbps to 0.5Mbps, especially after 5pm until about midnight. Upload speed off course is even worse.

8ta, has after months of me complaining agreed to install an antenna at my house, which will probably take another few more weeks for them to accomplish.

I often have 5/5 bars of signal on the router, so my argument is that signal strength might not be the problem. Since the connection is in a residential area, I would assume that their network is more likely to be a bit more congested after hours, but a technician explained to me that their network is never more than 30% "occupied".

Can the number of users connected on the 3G network/tower cause signal strength to attenuate for surrounding users?

Would it replace the B660 (7Mbps) router with a 21Mbps counterpart?