It may be worth checking with Solar if they will be updating their software to be MTD compliant, otherwise it will be mandatory to complete submissions via your software and you may need to explore a new solution in time for the implementation in April 2019.
There are several software solutions which currently use online VAT submissions including Pandle, which I imagine is very similar to Solar, also Freeagent, Quickbooks, Kashflow and Xero to name a few of the more well known companies.
There is an ongoing pilot scheme which 13 software companies are currently undertaking and Clear Books are even offering beta testing to their customers should you be interested in taking part.
There are also some companies who are choosing not to update for MTD for instance, VT Transactions who will rely on apps to submit data..it's a bit of an open book at the moment and hopefully after the pilot we will all be a bit clearer on who will be on board.
Hope this helps