You can indeed try to create a mobile proxy on your own. If you're not very tech-savvy and don't know where to start, try googling a detailed guide with step-by-step instructions. For Android, there are some free intermediary programs that create a tunnel between your phone and your computer, which can be used to make a proxy. Additionally, some open-source work is available on GitHub. Plus, you'll need to find a separate solution for implementing IP rotation.
If you decide to go this DIY route, I recommend you double-check the security and reliability of the programs used -- only use open-source solutions to avoid downloading a "Trojan proxy horse" that could end up stealing your traffic and doing who knows what with your SIM card.
Interestingly, however, the two services you tested gave you unstable results. Either you tried newcomers on the market, or the problem might be in your mobile network or device. I would recommend you consult with the support of these services to understand the reasons.