Modern and 2checkout works fine for us at least. The only problem is there is no recurring billing option so that simply will not work. Modern has said time and time again it will be in next version. Then they say it cannot be done what it seems to boil down to is they use Authorize so they will not do it. So if you need recurring with 2checkout it will not work sorry to say. We still use it and are satisfied with it minus this feature.
Well the option is you do not use recurring so each due date an invoice is generated and sent to client. Then incline they login to modernbill and pay the invoice. We have not had any complaints so far even though it is an extra step taken each month by client. The other option is pay someone that will change the scripts in modernbill so you can allow recurring. This is time consuming and probably cost you a pretty penny. So I say like we are doing get an Authroize.net account to fix it. You’re going to find with the high finance charges of 2checkout a monthly payment is not really worth it after transfer fees are taken out. So you could try whmautopilot I think 2checkout recurring is integrated with it. This obviously shows it can be done but modernbill is not doing it because of there strategic partnership with authorize.net. Best of luck on your decision and please keep us up to date as what you do.