This is a case for a Steuerberater (Tax Adviser), that has dealings with the Czech Republic!
From the (German) side, legally there will be no problems as long as you deal with
- the taxes etc. properly as a self employed person
What type of Aufenthaltstitel do you have?
The second is better when doing cross border activities.
As a general rule, National Visas have no effect in other EU Countries.
With a Daueraufenthalt-EU, anything you may need in the Czech Republic will be desided apon based on the rights granted by the Daueraufenthalt-EU. With the National Visa, from the Czech point of view, you are starting from scratch.
§ 9a AufenthG EU long-term residence permit
(1) The EU long-term residence permit is a permanent residence title. Section 9 (1), sentences 2 and 3 shall apply accordingly. In the absence of any provisions to the contrary in this Act, the EU long-term residence permit shall be equivalent to the permanent settlement permit.
(2) A foreigner shall be issued an EU long-term residence permit pursuant to Article 2 (b) of Directive 2003/109/EC if
1. he has resided in the federal territory with a residence title for five years,
Since the conditions for the issuing of a Permanent settlement permit (Niederlassungserlaubnis) for EU Blue Card holders differ
- having a Niederlassungserlaubnis does not mean that you are automatically qualified to receive a Daueraufenthalt-EU
- when the 5 years (60 months) condition is not fulfilled
§ 9 AufenthG Permanent settlement permit
(2) A foreigner shall be granted a permanent settlement permit if
1. he has held a temporary residence permit for five years,
3. he has paid compulsory or voluntary contributions into the statutory pension scheme for at least 60 months or furnishes evidence of an entitlement to comparable benefits from an insurance or pension scheme or from an insurance company; time off for the purposes of child care or nursing at home shall be duly taken into account
§ 19a AufenthG EU Blue Card
(6) Holders of an EU Blue Card must be issued a permanent settlement permit, if they have held a position of employment in line with subsection 1 for at least 33 months and have made mandatory or voluntary contributions to the statutory pension insurance scheme for that period, or if they furnish evidence of an entitlement to comparable benefits from an insurance or pension scheme or from an insurance company and if the requirements of Section 9 (2), sentence 1, nos. 2, 4 to 6, 8 and 9 are met and if they have basic German language skills. Section 9 (2) sentences 2 to 6 shall apply accordingly. The period referred to in sentence 1 shall be reduced to 21 months if the foreigner has a sufficient command of the German language.
§ 18b AufenthG Permanent settlement permit for for graduates of German universities
A foreigner who has successfully completed his studies at a state or state-recognised university or a comparable educational institution in the federal territory shall be granted a permanent settlement permit, if
he has held a residence title pursuant to Sections 18, 18a, 19a or 21 for two years,
he has a job commensurate with his degree,
he has paid compulsory or voluntary contributions into the statutory pension scheme for at least 24 months or furnishes evidence of an entitlement to comparable benefits from an insurance or pension scheme or from an insurance company, and
the requirements of Section 9 (2), sentence 1, nos. 2 and 4 to 9 are met; Section 9 (2), sentences 2 to 6 shall apply accordingly.