Anyone using Tandem had their credit limit increased? Mine’s still stuck (at the same rate as well) from the account opening limit.
With the other credit card providers (especially AMEX, so I’ve heard), they gradually increase it over time and in line with usage.
Could ask a CSR if they can increase your limit? They might be reluctant to do so because they don’t want to give away more cashback though 
We will be offering credit limit increases early-mid 2019. We are currently in the process of automating the process and will eventually be able to proactively offer it where suitable once we have built our own system for it. Sorry about the delay!