@Murali Shetty Thanks for your patience on this. I am summarizing the offline discussion and posting the same here.
As you are on developer SKU and the planned maintenance is ongoing you cannot upgrade/change the tier of your APIM instance from developer to Basic is not possible which is expected.
It is recommended not use developer SKU for production scenarios. only work around is to recreate a new APIM instance in Basic or higher SKU's and point your application to new APIM dns name.
In case, if you are using custom domain, you can update dns records to new apim.
Hope this helps, let me know if you have any further questions on this.
Please accept as Yes if the answer is helpful so that it can help others in the community.