Experienced PrestaShop developer needed to fix problems in existing installation

We are looking for an experienced PrestaShop developer to fix problems with our (not live) installation and make a few custom modules to work together. All offers will be considered but we will favor offers with a fixed price. We are looking the job to be done by the end of August. Here are the issues we need help with: 1. Unable to access back office after a period of time We have a problem where PrestaShop logs you off from the back office after some inactivity and you can't log back in - you are stuck in the login page. One workaround we’ve found is that you clear your browsing history but this is unacceptable for production use. The only explanation we've found is that it might have something to do with dynamic IP address (which is very common in our country). We have unchecked "Check the cookie's IP address" from Admin Preferences which was the most popular solution but it did not solve the issue. 2. Payment method modules not working properly We have two payment method modules, 1) Paytrail module which is a finnish bank & credit card payment processor 2) Invoice module Both of them are configured and active but neither of them are present on the payment methods page in the back office. Neither of them are present on the checkout page either so right now we have no working payment methods in the shop. We need an experienced dev to look into why these modules are not working properly. 3. Integrating one page checkout and custom order field modules to work properly with each other We are a B2B company and electronic invoicing is very popular in our country among businesses. We need custom fields to the checkout page so customers can type their invoicing email or electronic invoicing address. We have purchased "The Checkout" which is one page checkout module and "Custom Fields" module but they don't work well together. We want someone to move the fields added by “Custom Fields” module to sensible places on the 1-page checkout and to make sure that everything works correctly. For example, “Custom Field” module inserts some of the user created fields to the shopping cart page instead of checkout page. We want all the fields added by this module on the checkout page. Version info PrestaShop version 1.7.6.5 PHP version 7.0.33-1~dotdeb+8.1 MySQL version 5.7.30-log Debian version 4.9.189-3 (2019-09-02) x86_64 Custom modules referenced in this description The Checkout v3.2.9 - by Zelarg Custom Fields v2.2.0 - by FMM Modules Invoice Payment v3.0.6 - by silbersaiten Paytrail v1.3.0 - by 1st Iteration